We have play tested many different variants in round length. We found that the game worked well with the following.
Round 1 - 2 minutes
Round 2 - 1 minute, and
Round 3 - 30 seconds
By all means have a play around, but we would suggest keeping to the halving of the next round by the previous round.
If you want to add an extra round in so having two of each round, for even 3. Do so based on the size of the team.