There is no hard and fast rule for wheel nuts; it depends entirely on the design of the wheels. I have Fiat alloys which fit on my Stilo using the same bolts as I used for the original steel wheels.. but my old Alfa 155 needed a new set of bolts for its alloys. The difference is that my Fiat alloys had quite a deep bolt hole (with a cover over them) and the Alfa alloys had a flat/barely recessed face on it.
The only way to tell is to take a wheel off and push the bolt through the hole. It might stick out 25-30mm or thereabouts. Compare that to the alloys.
In any casem fit an alloy onto the car and tighten the bolts up. If the bolts are too long/stick out too far they will pass through the hub and poke out the back, where they might foul on the brake caliper bracket or suspension arms. Try the same thing on the rear too (since the setup is different, particularly if you have drum brakes).
If the wheels rotate without fouling then that's cool.
The rule of thumb for how much thread is enough is 1.5 times the length of the bolt diameter... so 18mm of bolt poking out the back is the bare minimum... but you might be able to see the thickness of the threaded hole (in the hub). That's the minimum length of thread length you need.
Ralf S.
