NauOne
There's a commonly accepted timeline:
864 ABY
But I don't think anyone would really force you to write in this timeline. Though, I don't know anyone who roleplays outside of this timeline (except for those netherworld people), so it might be hard for you to find partners or join other threads outside of the generally accepted timeline
Edit: As for Legends or Canon, I think everyone makes a melting pot of what they want. I lean more along the Legends, and don't mind technology from Canon existing, but the newer Canon timeline can get a bit muddy especially since CHAOS seems to have branched off from Legends.