IMO/IME, you can have it as long as you want. Generally speaking you don't want the handguard covering the muzzle device. I'd go for no longer than about 12". Especially with it being free float.
If it is free floated you can use anything out to 12" or possibly a 14" as long as you are using a low profile gas block. So a carbine or mid-length handguard would work fine.