How many millimeters of space between front teeth would there be for a dentist to look into closing the gap?

That Depends: I have found that this is a personal choice for the patient. If there has always been a diastema between the teeth then i try to keep it. However with the history of srp and lanap you have a periodontal issue here. The loss of bone support has allowed your teeth to splay. I would be less likely to keep the gap and more focused on splinting your teeth for strength. Save what you can now.

Diastema: The cause of the gap, the occlusion (bite), the symmetry of the teeth, the harmony of the size of the teeth, the smile line and the wishes of the patient all play a role in determining not only whether to, but also how to close the gap. Not all gaps need to or should be closed. This is a very individual decision based upon the specific circumstances. See a dentist to determine and discuss options.