Is there a rule?
First, you are at -4 to hit because it is an improvised weapon.
After that, you might do 1d6 + 1d6 fire, BUT
you'd be within your rights as DM to have the torch break if swung at full force by someone with a decent strength bonus (+3 or more str mod). I'd probably even throw in a % chance that it just breaks even if wielded by a wimp.
Normal melee weapons don't break, (unless consciously and convincingly sundered by an opponent,) that's how they're made. The average torch maker doesn't expect you to swing your torch at hardened steel shields, beat past swords or otherwise use it for weapon-like purposes.