For pure aesthetics, how much should I spend of my discretionary income?

What you can afford. This is a great question but with no answer. This comes down to what you can afford. If you need help to set a budget there are a variety of resources but aesthetics are hardly an essential item unless your career depends on it and what takes priority in your life. Excellent skin care can be accomplished with relatively modest means.
Money for aesthetics. You should set up a budghet, decide what bothers you the most, then get a consult to see if the work can be done within your