Prob 30
You sound worried by the prospect of 15 days away a month...
Remember that many of your intial B777 trips will be '3 days'. What that really means is: Day 1 report early afternoon(ish). Day 2 Away. Day 3 Land back at LHR fairly early in the morning. You might need a couple of hours sleep but the rest of the day is yours.
So a 3-day trip is actually about 48hrs away from base. One night in a nice hotel and another in the flight deck where you are 'encouraged to rest'. In practice you'll each get about an hour of shut-eye if things are quiet.
I don't have kids (yet) but I think it would be a great lifestyle as a dad.
Come on in, the water's lovely.
LFW