How long should gelcoat take to cure?

[MarkB]

To do repairs like that you should either use top coat which is sometimes called flow coat or add a percentage of wax solution to the normal gel coat. It will never cure hard.
Your best bet is to dig out the gel then redo it but put some thing like acetate or anything plastic that won't stick to it to seal it so the air can't get to it........Or just use resin pigmented to suit as this will cure and is easy to flat off and polish up.