ramp-up "guard" time (data receiver)
The main purpose of the "guard time" is to give the the receiver's AGC the time it needs to adapt to the new carrier level and allow the data demodulator to begin operating correctly.

Many AGCs have a fast-attack, slow-decay characteristic. The preamble of a packet generally starts with a data-free zone that allows time for the fast-attack phase to work &mdash; i.e., when starting from a "no signal" condition.

But the slow-decay phase requires a certain amount of additional time between packets, which is the "guard time" you're talking about. It is primarily of importance when a weak transmission (from the receiver's point of view) immediately follows a strong transmission. This is just one aspect of the near-far problem in wireless networks.

If your receiver doesn't give you specific values related to the time constants of its AGC, you may need to do some experimentation. But as glen_geek suggested in a comment, you should be able to accomplish this with a binary search, rather than an exhaustive sequential search.