How do I know when I need to upgrade?
If you don't know, then you don't need to upgrade unless the camera body breaks. Besides, when upgrading, you'll find there are multiple camera bodies available that could fulfill your needs, and you need to know which of those bodies is the right for you. If you don't know, then don't upgrade.
I have a complement of "kit" lenses, such as an 18-55mm f3.5-5.6 and a 55-200mm f4-5.6. Maybe I've even added a 50mm 1.8 to the kit. Flash is only for indoors (right?) and I usually shoot outside, so that means I don't need an external flash.
Those lenses aren't particularly sharp, so if upgrading to a high-megapixel body, you'll find you need to upgrade your lenses too or else you won't get the benefit from megapixels. Some alternative reasons to upgrade could be better autofocus, low-light performance, faster burst rate, etc.
I shoot photos of my family and friends whether out at the park or having a picnic, my kids sports practices and games, vacation photos, as well as some landscapes, flowers, and whatever else catches my eye. Pretty familiar territory. I'm not a professional and don't intend to become one.
It sounds like you don't need the low-light performance or burst rate that better bodies could give you. So, upgrade when something breaks.
About the only indication I found that you might want to consider upgrading at some point of time is shooting sports. Look at whether you can catch the decisive moment in your sports photos. If not, a faster fps body with fast card support (and the fast card obviously, too), could help. Look at whether the sports photos show excessive motion blur. If so, you could benefit from faster shutter speed which in indoor light conditions requires high ISO (and thus a new camera body with better low-light capabilities, perhaps) and fast aperture (and thus perhaps a new fast lens, too). Since your current lenses are zoom lenses, you should have a good idea of what focal lengths you need for sports. But, if the sports you shoot is outdoor sports, perhaps your current lenses and camera bodies are enough.
I'll give an example of my crop body upgrades that I think were justified.
I first bought Canon 2000D when starting photography. I found it's an ok camera for many situations, but not for all. In particular, I found I enjoyed bird photography buy found myself limited by slow burst rate (even slower when the tiny buffer is full), poor autofocus and no "stop AF search" option. The higher weight of many of the more expensive crop sensor bodies made me think hard whether an upgrade is what I need.
Then I found a reasonably priced used Canon 70D. The reasons I bought it were "stop AF search" option, 7fps burst speed and 19-point autofocus that can in some cases be used for flying birds too. As a minor improvement, the 97% coverage viewfinder made aiming long glass slightly easier than the 95% coverage viewfinder of 2000D. I also found the button to switch between different counts of AF points very useful, which I couldn't anticipate before buying the 70D and using it in the real life. Testing revealed that with proper technique of carrying and holding the camera, higher weight can be tolerated, and the weight of a reasonable long lens is anyway higher than the weight of the camera body.
However, I also found that even if I attach a fast SD card, when the buffer is full the RAW burst rate is not much faster than one shot per second. Disabling JPEG and shooting only RAW could help, but only in a very minor way. Shooting only JPEG could help at the tremendous cost of low dynamic range in underexposed shots.
The solution which I haven't tested yet was to sell the used 70D (at a loss of bit over 100 EUR, probably less than renting it would have costed -- we don't have a functioning camera/lens rental service where I live), and buy a new 90D. The 90D maintains the autofocus, burst and "stop AF search" benefits of 70D, but has added some crucial features, in the order of increasing importance from first to last:
- Lossily compressed C-RAW support enables 32.5 megapixel resolution at a smaller size RAW that a 20 megapixel camera would shoot
- The buffer is larger than on 70D
- Support for fast memory cards enables fast shooting speed when the buffer is full. The small C-RAW format helps here, too.
There are also some features that weren't the deciding factors for upgrade, but could be useful, in no particular order:
- 10fps as opposed to 7fps
- 45-point autofocus as opposed to 19-point autofocus
- 32.5 megapixels as opposed to around 20 megapixels, although now the resolution is probably limited by glass rather than by sensor
- 100% coverage viewfinder should enable slightly easier aiming of long glass than 97% coverage viewfinder
- Weight has been slightly reduced
- Shutter lifetime has been slightly increased
Soon, when the risk of rain will be lower than it is today, I'll see whether the 90D was a justified purchase.