Leo doesn't think there's a general rule, and it really depends on what each user is comfortable with. That being said, Leo says he thinks users can probably do more than they might think, and he usually takes the risk. A good way to gauge the difficulty level of doing something is by watching a video showing how it's done. If it looks too daunting, don't do it.
A great place to go is iFixIt.com. The whole point of that site is to be the 'repair manual of the internet'.