I got my Artley in 7th grade, so my memory is very likely fuzzy on this. But I just don't recall having any issues learning picc. Then again, my band teacher was a flutist herself, so she likely gave me tips. And I have always switched freely between picc and flute (she taught me to switch to flute on long rests in the picc part), which probably makes things way easier.
I'll try to remember to ask tomorrow night at Band--there are at least three others who play picc. That should give you a nice average of how long it takes to be fully comfortable on the picc.
Flutes:1975 Gemeinhardt M2 in chrome nickel;1982 Armstrong 80;2006 Yamaha 584
Piccs:1978 Artley piece of crap 15 P;1982 Gemeinhardt 4S;1980s? Armstrong all wood (no model)
Bass:2006 Jupiter di Medici G0199
Treble:20?? Guo New Voice
+ many flute-cousins