It really can't become *too* slushy. You can ski on all of it until it's melted. (How enjoyable it is can be debated).

When you're skiing in mud, it's too slushy.

That depends out west (u.s.a.) in high altitude it can be very warm (45*F) and snow be ok here in the east sunny and 40* for a few days in a row it will seem like you are skiing on mashed potatoes there many variables humidity, base depth, pitch, night temps ect.. but generaly speaking I would say up to 40*F your ok

3 or 4 *c