24 seconds I think. What I did was:
1. Divide 60 by 5 to get what 1/5 of a minute is
2. That would be 0.5 cm in 12 seconds
3. To go from 0.5 cm to 1 cm we multiply by two
4. To keep it balanced we also multiply the time: 12*2= 24 seconds
So it would take the snail 24 seconds to move a centimeter.
If I'm wrong I apologize and I accept any corrections