Whats the investment horizon for sti etf to be 'profitable' using dollar cost averaging for both nikko and spdr?

There's absolutely no guarantee that any investment will rise in the future but it is likely.
I don't have figures for the Straits Times Index, so I'm using the MSCI Singapore index instead.

As you can see, since 1970, the worst return over a 15 year period is positive. If you have held the MSCI Singapore Index at any one point in time for at least 15 years, you wouldn't have lost money.
This would be somewhat similar to any individual country equity index and most probably a world index as well.
You should plan to always hold any equties for at least 10-15 years before even considering investing in it.