I have hit my Medisave cap and my SA has 60k. How Long more must I continue to do a voluntary SA top up to reach the FRS and let it compound?is it true that if I contribute in January it is better?

You can only perform a voluntary top up to your special account as long as you have not reached the full retirement sum (FRS). How long more to contribute really depends how much you contribute to your special account each year. It is indeed better to contribute in January due to the compounding effects.

As long as your FRS has not hit the cap in the current year (e.g. 2020 FRS is $181k), you can still continue to contribute via SA topups. Making contribution as early as possible is better, not necessarily in January. However, if you are aiming to qualify for tax relief and have already utilized the $7k relief in the current year, then next year January would be the next best time to do your contributions.