Actually there is no natural level to exit. Fighter and eldritch knight get nice things every level. 6, 7, 8, 9, 10, 11, 12 are all good. So just switch whenever you like. You can always switch back. Actually you will delay your high level spells known with wizards. So maybe exiting fighter earlier is better and take more fighter levels later after you get to the desired wizard level... 5, 6 or 7 come to mind. Considering that you are level 7 already you will ve level 12 overall when you finally get level 3 spells. Only one level earlier than when you would normally get level 3 spells known. You will have level 3 slots earlier but that won't give you the real flashy spells.
So if you still plan to be a wizard exiting fighter now at level 7 is the latest level that makes sense if you want to take wizard to be a more powerful spellcaster. If you take wizard for versatility you can always take a short break from your fighter progression.