I know this doesn't sound like a very helpful answer.

But, too much is what is too much for you.

It is very different for different people and for the same person at different times.

How can you know what is too much for you?

By trying and finding out.

As a rule of thumb, always do a little less than you think you can.

Don't criticize yourself for not doing enough, or for doing too much.

And gradually find the optimal balance.

Using one muscle group may (or may not) lead to fatigue of other muscles.
This depends on the type of MG and possibly its severity.

Patients with purely ocular myasthenia, for instance, will not experience fatigue of other muscles no matter how much they use their eyes.

Some MGers will only experience fatigue during exercise and fully recover with rest, other will have on-going symptoms for the next few days.

As I have said before, you have to gradually (and cautiously) learn the pattern of your own illness. And also learn to recognize any significant changes.