That all depends on how late you pitch up and how early you leave, National Trust wardens, Police and Council employees are all over it in the summer.
Personally as a Cornishman I avoid St Ives, mainly because it is packed and impossible to park.
I use this car park but the rules seem to change with the wind, generally free but occasionally an overnight charge. A seriously beautiful beach as well.
Harveys Towans Car Park
★★★★☆ · Car Park
goo.gl
EDIT: don’t even attempt it in a lowered van, the track in is as rough as a badgers arse