You should start planning your renovation as soon as you receive your floorplan for resale units. For BTO units, you can start planning anywhere between 6 months to 1 year prior to getting your keys.
Planning helps you to:
Get a sense of the cost of your initial intended renovation works
Enough time to look around for a suitable ID
Compare quotes and get different ID proposals (after all you will be spending 5 to 6 digits on renovation)