Finally found the formula!
The number of minutes M necessary to get back to a good condition after an absence of
D days, is simply: M=17*ln(D)*X
(usually X is 1, but it depends on your age)

So, if you're away for:
2 days => 12 minutes!
15 days => 46 minutes!
6 months => 89 minutes!
1 year => 100 minutes!
10 years => 139 minutes!

All those number, assuming you've mastered Wipeout HD in the past, otherwise the curve
is drastically different!