Is it correct that I should only sell my STI ETF (with DBS RSP) when I need the cash and not when the market slumps?

Yes correct or another reason is if you lose conviction and find a better alternative, then that's when u should switch. For myself I switched from super terrible index (STI) to arkk etf and never turned back again, no regrets

Holding on to it long-term is likely to maximise your gains because you are riding out the ups & downs.

If you need cash in the short-term, ETFs may be risky as it's not certain whether prices are up/down/positive/negative.

Imo, this train of thought seems abit flawed yet a wide belief.

let’s think in terms of opportunity cost, if there are better alternatives, ppl usually dont pursue the better alternative because (1) fear of unknown (2) habits (3) not knowing why they buy in the first place (4) not knowing about other alternatives.

another thing would be that if you had to sell when you need the cash to solve a problem, and it’s during a market slump, would it be correct to sell? similarly, if during a market slump and there are opportunities, and you “need” the cash, is it still wrong to sell? rmb, we are never in control of the price.

it all boils down to having a plan, sticking to that plan, doing due diligence, experience, patience, courage and luck. a mixture of everything actually.

so in that sense, everyone has different investment timeframe, because only you will know what sort of financial goals you want and commitments you have, and the plan to get there. things always change btw, so prepare for that.

*oh and I realised this is quite an old post, literally 1 year ago. but still relevant I guess to anyone reading this given what happened in 2020.

I think, when you wish to sell really depends on why you invested in the first place.

Generally, if you're investing for retirement, it is best to leave it be and if you need the cash, as far as possible, try to get it from your income or savings first. This is because, as the saying goes, "Time in the market, beats timing the market." By taking your money out, you're disrupting the compounding effect and reducing the amount of time your money has to grow.