I own a Canon 550D, and I'm planning to do some time-lapse experiments this weekend just for fun. Will it cause any damage or reduce my DSLR's lifetime if I do too much of it?
The camera's shutter has usually a rating for 50,000 to 300,000 actuations, depending on the camera. For yours, I think it is closer to 50,000 but you should check the manual to be sure.
That means you can take that many pictures and still be within the manufacturer's life-expectancy for your camera. When you do time-lapse, you take a lot of pictures, so you are going to eat away at that number. How much depends on the time-lapse you expect taking.
That being said, the number of actuations is just the manufacturer's educated guess. It can fail before or after, I have heard of people who managed to shoot 4 times more before the camera's performance became erratic. If it does fail before and you are within the warranty period, you can probably have it fixed at no charge.
I thought I'd offer a completely different, perhaps complementary way to look at this question: There's more risk to more than 99% of the camera users out there of not using their cameras than there is of ever wearing their cameras out. Use your camera! Break it if you have to! But get it out of the case and learn something! Don't get hung up on what you're going to do to the camera -- it's replaceable, Canon has a big factory -- if you wear yours out, they can make another one. Give yourself a real reason to buy the next big thing from Canon in two years.
Worry about doing this project right -- worry about how much you're going to learn from this, worry about getting the very most from your data and the time/effort it takes to do the set up right ... do a couple smaller trial runs to ensure that you are going to collect the best data possible.
Finally, look at the economics. You can purchase a Canon T2i/550D camera body replacement for less than $700 (you won't have to; Canon will likely give you another if you wear it out doing this) $700 is less than a penny per actuation @ the quoted 100,000 actuation rating. Your time to really review, process, edit and really get the most from the data from 100,000 actuation is probably worth several times more $700 ... if you spend just 5 seconds on average per actuation/exposure processing your data, you'd spend 500,000 seconds ... at 8 hours/day of full-time data analysis and editing, that's almost a full month of utilizing your photographic data ... my assumption is that your time is worth at least 10X more than what the camera body costs -- if I can assume that your time was what supplied the income the supported the purchase of a discretionary item like this DSLR.
[NOTE: As an engineer who's been involved in launching/testing similar products; I'd be slightly surprised if your camera wasn't good for 250,000 or even 400,000 actuations if it's rated for 100,000 ... unless you've managed to somehow introduce some extremely fine powder or some other extra wear agent into the mechanism ... we typically test-to-failure and try to eliminate failure modes in the early failures; you be surprised how long it takes to get the last 95% of the population to fail ... yes, your camera could fail early; that's what warranties are for ... regardless of what the warranty says, the company is probably going to treat an avid user very well if the camera fails in the first couple months, i.e. Canon's cost per unit might not be zero, but it costs Canon a LOT, LOT less than $700 to replace the camera and buy an avid fan for life. Canon understands the DSLR market share game well enough not be stingy on warranty replacements.]
Frequent shooting of the time-lapse videos will eventually wear your shutter, and there is a way to limit that "side effect" other than replacing the shutter when it finally breaks. Remember that time-lapse is meant to be presented only by means of digital displays, like computer screen or a projector, not to be printed, so you don't need the full XX MP resolution, Full HD will always be fine enough. So, instead of actually shooting your frames, film them! If a session is not planned to take longer than a few hours, use one or several high-capacity memory cards and then decimate your frames on a PC using one of the widely-available movie editing apps (by e.g. dropping 9 out of every 10 frames to get the actual 3 FPS). If the recording session should be longer, like nature slow-motion, and you're a lucky Canon DSLR owner, use Magic Lantern alternative firmware — it has a slow motion mode that you can set to anything down to 0.3 FPS, which is equivalent to shooting one picture every 3 seconds. The mirror is locked and the shutter remains open for the whole session, voila!
The only think I could add to Itai's answer is that the shutter failing after X actuations doesn't mean the end of the camera, you can have the shutter replaced for around $250 at a Canon authorized repair centre.
Bridge and compact cameras with electronic shutters are more suited to timelapses however, for this reason. An electronic shutter uses circuitry to stop/start the exposure meaning there are no moving parts. The number of images you can capture is practically limitless - think about how many individual frames a digital video camera will produce in its lifetime!
Let me spin it another way to worry you. A prosumer Nikon D7000 shutter is rated for 150,000 actuations. At 24 cinematic fps this is less than 2 hours of "footage".
But, this is a hell of a lot of interval footage (just ask Ron Fricke!). Feature length time-lapse productions are rare and specific. This fact, combined with the likelihood that YOUR shutter will live much longer works in your favor. Or you could be unlucky!
Tim Burton and Nick Park, on the other hand, shooting on the opposite side of the interval spectrum would probably go through your camera many times over in a stop-motion production. But we're not worried...they could afford it. ;)