When is the best time to sell off my BTO after MOP?

Once the MOP hits, that will be the best price!

Ideally, as soon as you can after you reach the MOP. This is to ensure you capture the full value between the resale price and the price at which you bought it. At any time afterward, the flat will usually see some depreciation and be subject to secondry market forces.

The challenge is not that it's hard to sell your flat soon after MOP though. The challenge is in having an alternative. What is the next property purchase that works for you? If the only next option is another resale flat, then maybe just hang on to your BTO a while.