when do I have to change the timing belt in a Hyundai atos 1999

The typical replacement interval is every 60,000 miles/96,000 kilometres

every 60,000 miles or you risk the belt breaking and the engine getting damaged.

60,000. And by all means use a genuine Hyundai belt.

I've replaced them with a Gates belt on 2 different cars and the engine would vibrate and sound louder. Customer is came back and l installed a genuine Hyundai timing belt an both of them and all was well.