I think this is more of a personal preference type thing. Sometimes I will just put the link to MathJax, and sometimes I will edit if I am in the mood for it. So in this answer I will just discuss what goes into my decision. To be clear, these are my personal preferences, I am not citing any Physics SE policies here.
- Is the question on topic? For example, if the post is just an off-topic homework problem that has text variables and equations, then the most I will do, if anything, is to give the MathJax tutorial link in a comment. I don't think it is worth the time to edit a question that will be closed anyway. In contrast, if the question looks like an excellent question, but the lack of formatting is getting in the way of the readability, comprehension, etc., then I am much more likely to do the edits myself in addition to supplying the MathJax tutorial link. I might say something like
Please use MathJax in the future when formatting variables and equations. I have edited your question this time as an example. Please make sure I have correctly interpreted the equations you were using.
- How much editing needs to be done? If there is a lot of equations to format, then I am less likely to edit than if there just one or two equations are needing to be formatted. In cases where there are many equations, I might just edit one and then leave a comment talking about MathJax and saying how I edited the first equation as an example.
- How generous am I feeling? Sometimes I am just in the mood to edit, other times I am not. That can influence how much I end up editing, or if I choose to comment (or even just move along and not do anything).
My final decision of whether or not to edit and how much to edit is usually decided by the three above questions. Therefore, I don't really have any "hard and fast" rules on when to edit, but this is how my general thought process goes.
I will say that I do agree with your sentiment of making sure the OP learns how to use MathJax (especially if they are going to regularly post on the site), but this is a harder thing to control or consider because there is no way to know if the OP will even come back to the question, how much time they will put into editing their own question, and how much time they will put into learning how to use MathJax. So this thought doesn't really influence my decision here. If the OP is one to put the time into MathJax, then whether or not I edit the question will have little bearing on what the OP ends up doing.