That's the problem, exactly as the above poster said, it depends on when your final sim is scheduled. If you're lucky you could get your first paycheck after 7 - 8 weeks, if you're unlucky could be 3 1/2 months.
Also bear in mind that flight pay is paid a month in arrears so your first paycheck is not going to have any flight pay in it, it will be a few hundred quid less than you might have otherwise expected.