10 or 20 depending on the ISP's chosen DLM profile.
That's assuming full 24 hours uptime which it won't be as it takes time to resync so probably 9 or 19 allowed per 24 hours.
Being scared to reboot the modem once in 3 month period is crazy as you could do it once a day and not bring it to DLM's attention.
Why do you need to?
Is the first question...
Memory usage increases over time.
Actual SNRM decreases over time (quite a bit on some modems).
I reboot my modem about once a month. It's the only resync my line has over the month so I don't bother waiting 30 mins as DLM isn't gong to care about a single resync.
As B*cat pointed out if you want to make sure DLM doesn't even count a resync then leave a full 30 mins between dropping sync and establishing sync again.
DLM works with 15 minute bins and if it sees 0 uptime for the 15 minute bin after the resync it will assume it was an unforced event and discount it.
https://kitz.co.uk/adsl/DLM.htm#MTBRAs you have 2 lines you could power 1 off for 30 mins. Reconnect it as you power off the 2nd. Bring that back up 30 minutes later. Then you have no downtime. That's if you're super paranoid about DLM that is
Personally I would just reboot them quickly.
If you want to get the highest possible sync then reboot them when the SNRM is at its highest. Some lines SNRM can vary by 1dB+ between morning and night and the attainable goes up and down with this.
Do your lines cause each other crosstalk? If so do you have an order in which you sync them?