It is actually pretty easy to get a pretty okay number, count ur gens, add the xp u did, minus double xp nshit, add the xp ur chars have now.
Then ull get a sick amout of xp. Divide it now with the average amount of x you have, like 2,5 should be okay for most. Will be then for me something like
600 mio : 6000 xp (joined right when they enabled double xp)
= 100000
100 000 : 60
1,6 k hours.