@Darkwing : I took some guides and advertises everywhere here and on the net. So I will add how I did it
For the time spent it's difficult to say. Magnetization itself it's really less important than assembling and painting miniatures.
For twelve minis I think I spent 1h to drill and stuck the magnet. For corresponding weapons maybe 2h. First try was long but after some practice it's quick (compared to other work).
Of course I stopped playing video game in the same time to avoid wife issue
@Istvan : I used 1,5x0,5mm (power N50, 20gr adherence) for wrist and 2x1mm for back (N50, 100gr).
@Sid : I didn't have trouble during my latest 10h played with the magnetized minis. Some weapons like thunder maul or dragon slayer could rotate if not blocked by another parts of the mini and in this case the back fitting works like a charm. And have the others players crying like a little girl each time he has a new weapon or armor to equip is really grateful
For the magnetized paper it's normally dedicated to printer (A4 format) is expensive for my use (11€ for 3 sheets) and Maybe your will be better.
For me it's necessary to have this kind of parts stockied like that to avoid to spend too many time in search during playing.
Edit for Istvan latest answer : by mistake I drill 4 minis at the 2mm diameter ... It's really just but it's feasible.