Is there a general rule of thumb for how long to wait before judging whether a new combination of medications is working?

Thats going to depend on the medication. Non stimulant medication (like atomoxetine) needs to build up, to cross the blood brain barrier and then effect the brain. their is always a lag of up to 4 weeks. For stimulant medication, we can know quite quickly. the medication is processed quite quickly by your body, but the metabolites hang round for a bit (eg in lisdexamfetamine) , so it can take us a few days to work out if something, or a change is is working. Sometimes up to two weeks to allow for a psychological adjustment too. ask your clinician this question specific to your circumstances, they can make a care plan with you