I guess what you need is to know in advance how much time, effort, and earnings will involve a gig. Not just from Fiverr, but for anything. Maybe whay helps is

I guess what you need is to know in advance how much time, effort, and earnings will involve a gig. Not just from Fiverr, but for anything. Maybe whay helps is to timebox the assignment: I will be available for fixes/etc for... a week, maybe two weeks ? Also, having a ticketing system might help you: it is very too easy to send an email from the phone to ask for new stuff and you need to track time anyway.