first off it's SKIS, not skies, not ski's, not skiis, not skiiieieeaieirsais, sorry, it's a pet peeve of mine
and for my race skis i tune like every 3 or 4 days depending on when i have time and freeride skis i tune like once a month
When I start noticing my speed dropping, and just to keep them in good condition 1 or 2 times a week when skiing, and leave thick layer of wax on over the sumer or when not using them for a bit.