There isn't a set in stone procedure other than playing. They will mellow after a few hours of good high volume use as the surround material loosens up.
I recall my brother “breaking in” some speakers once. I think he was pissed at something…he simply kicked in these two nice speakers in his living room.
I've built a few sets of home audio speaker/cabs. The general rule of thumb on those is around 4-8 hours to help get things loosened up. You don't seem to get full/smooth bass response untill they are fully broken-in.
On guitar - I'm not sure that it would be very long at all. Probably around an hour.
For bass/PA applications, I'd look at 4-8 hours before I really started pushing loud/deep bass.
In reality, all speakers are probably ok after about 5 minutes