I'm using Nikon D750. Is there a time limit as to how long we can have bulb mode on?


You can use the time exposure mode as long as you want - at least, until your power source is exhausted. You'll gradually get more noise as the shutter is open longer, from the sensor heating up, so it's seldom done to such extremes. (In the film camera days, very very long exposures, while uncommon, were done much more often than they are done today, e.g. for star trails.)

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    I'd add that regularly doing long exposures (of the order of tens of minutes) can "hurt" the sensor in the sense that it can develop hot pixels that are then easier and easier to see in "normal" conditions.
    – K. Minkov
    Jul 20 '18 at 7:41
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    I agree with the "until the battery runs out" entirely. As for long exposure being a problem the longer your exposure. That is true as well but I would not hesitate to do it if you needed to. In my film days I would regularly do 2, 8, even 16 minute exposures for night time city work. My D700 can handle that no problem at all. Newer bodies will likely do even better.
    – Ian Lelsie
    Jul 20 '18 at 14:14
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    @IanLelsie When I was referring to long exposures, I meant multi-hour ones - I am not sure I'd do that on digital, but multi-minute seem okay. Jul 20 '18 at 15:14
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    @Tetsujin On a practical level, it simply means that the electronics get warm, and when this happens, it's more likely that pixels will register light hitting them that didn't actually happen, creating noise in the image. At an extreme point, it's possible to do damage but I don't think this is ordinarily a concern, otherwise there would be more warnings in camera manuals about it. Jul 20 '18 at 17:11
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    @JimMacKenzie - noted. ;) I probably will never need to... just the potential for 'vague information being scary' is an easy interwebz trap to fall into.
    – Tetsujin
    Jul 20 '18 at 17:19

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