Officially it's half an hour before departure, but when a flight is full they close it once everybody has checked in, which is usually around T-40 as @
jerry_greece wrote, and sometimes even earlier.
Beware though that according to my experience while switching from EB to M&B the FFP number you enter during check-in (sometimes even during booking) tends to be quite ...persistent, to say the least. It happened to me that miles from some flights (with various airlines) credited to my EB account although I changed to the FFP number to M&B at some point before boarding. But that was several years ago (around 2013 I think? during another turbulent period for SK) so things may have changed now.
If your goal is also to enjoy your *G benefits (extra bag, lounge, priority, etc...) I would recommend that you enter your M&B number, and present your M&S card when required (check-in, lounge, boarding, etc). I am not sure how this will play out for op ups, but I can tell you that as an A3*G I haven't got one on an A3 flight in about two years, and I fly around 40 segments per year with them. M&B status pax receive a number of complimentary upgrade vouchers which reduces availability in J. Recently A3 starting auctioning upgrades online which decreased availability even further. Adding to that that during summer months flights to/from Greece tend to be quite full, I would guess that your chances for an op-up are quite slim (but again, that's only my conjecture, I may well be wrong). I suppose your optimal decision depends on how badly you need those M&B miles.
Note also that A3 offers a euro-biz product, meaning that there is no bulkhead, just a movable divider. Which implies that they would understandably rather sell six more revenue Y tickets than upgrade for free a couple of pax.