I am definately not an expert but I will try and answer your question.

The answer to your question will depend on at least two factors. 1. Your charge rate and 2. How much capacity did your plane use on the three or four flights.

A safe charge rate for a 600 mah battery is around 60 mah (10% of the battery capacity). I would venture to guess you have about half of the 600 mah left in your battery so you will have to charge back 300 mah to get back to a full charge of 600 mah. When charging a battery, you don't get a great return on your charge investement, in other words, to get a 300 mah into your battery, you will have to charge in roughly 380 mah. So if we take the 380 mah and divide by the assumed charge rate of 60 mah, you will require to charge your battery for 6 hours and 20 minutes. I would just charge it over night ( 8 hours) and be confident you have a full charge (this will also give back any capacity you lost with the battery siting for the seven days).