Can my son and girlfriend extend their stay in Thailand after arriving on a visa exemption?

Hello.

My son and gf are coming to visit in December and would like to stay with me for 40-50 days. They both travel on South African passports and usually get a 30 day visa exempt entry on arrival. They would like to apply for an extension at immigration in Phuket once they’ve arrived. My questions are:

1. Any idea how long will the extension be for - another 30 days?

2. How soon after they arrive can they go to get their extension?

I am trying to save the airfare from Cape Town to Johannesburg to get a 60day visa prior to arrival as it will also entail an overnight stay.

Thank you kindly in advance 🤗