For DBS RSP for STI ETFs, how often should I check how the stocks are doing?

You can even pass them to your grandkids.

Don't have to track at all hahah. Stocks are meant to be held for the long term! You can just take a look at it once per year, that is actually more then enough.

Do DCA for ETF and check it once every year max.

There is very little need to monitor on such ETFs, even less than once a year is doable