Simply, the eariler the better, perferrible at the time of booking if possible. There are many benefits to buying your policy right away.
1. In specific cases you have the advantage of insurance coverage for the longest possible amount of time.
2. If you book your trip and buy the travel insurance immediately, you are covered for trip cancellation.
example:
You want to take your family to Paris. You book your vacation as earily as possiible (for this example 8 months out) You would have the coverage and peace of mind for 8 months. If you booked a trip now then waited to purchase travel insurance, something could/might happen between the booking and traveling date.
3. Many plans include extra benefits if the plan is purchased within a certain numbers of day of booking.
4. Most policies have a grace period for purchasing where you can still take advantage of these additional coverages.