Diplomacy is a long game. You want to enforce time limits in order to keep the game moving at a good pace.
Time limit: 10 minutes is on the short side. I would recommend starting with 15 minutes and altering it as necessary according to how the players feel about the lengths.
Don't separate negotiation and writing down phases: 15 minutes combined for negotiation and writing down orders. These should NOT be two separate phases. Most established FtF diplomacy communities have moved away from having a separate order writing phase. The reasons for this are:
- Having separate writing phases drags out the game length significantly (and there's nothing worse than someone with "decision paralysis" taking forever and wasting everyone's time.
- Experienced players will talk during the separate order writing time anyway.
- There's nothing more boring than sitting waiting for 5 minutes for someone to decide what to do.
Most tournaments are run on a continuous clock, where deadlines are (say) 17 minutes for spring and 15 minutes for fall. These times include adjudication of the previous round + retreats + adjustment, so you're really playing a game year every 32 minutes. I consider 16/14 timing fast and 18/16 timing leisurely (x/y = x minutes for spring, y minutes for fall) in a tournament situation.