I have waxed clients after 3 weeks, I don't like to do it because I don't think the result is generally very good, but sometimes someone will need say a bikini wax before a holiday and the timing has got out of sync. Under these circumstances I would take out any really sort ones with pointed tweezers. Ouch!
It is a good habit, when re-booking a client, to ask them if they have any holidays or special occasions coming up. Then you can plan their appts over 2/3/4 months and make sure they have optimum hair growth when it matters most.