Whats the shortest contract could prior service do for commissioning in the guard?

The Guard was doing a "Try One" contract, don't know if that is around anymore. The Guard commissions officers through three different versions, each with a different duration. The "Traditional" Guard OCS is a 14 to 16 month process that includes two AT periods, usually commissioning on the the second one. Most states also have some slots available to Federal OCS, which is 90 days at Ft. Benning. Some states will also allow you to take an Accelerated OCS which is 8 weeks, but the dropout rate is high and some states just don't bother with it. In any of these cases, by the time you get in and through the entry process to OCS, a couple of years would be about as short a period as I would think you would need to get it done.

Remember that after commissioning that you are required to complete a Basic Leadership Course for the Branch you commission, with 20 to 24 weeks Active Duty about average for most Branches.

Assuming that you have all the required punches in your card, OCS is the quickest way to become an officer (and the only way after 24 years of age other than Direct Commission) afterwards that you will need to branch (Functional area) qualify. Irrespective, the last I heard of the service requirements it was still basically 8 years for officers ...
3 active, 5 reserve/ARNG
5 Active
8 Reserve
Things change so I may not be up to date on this.