Leave for active duty accrues at 2.5 days/month. You can have a maximum of 60 days carry over to the next FY (which starts in Oct and ends in Sept).
If you take leave over a weekend, it counts against your balance. In civvy world, if you take a long Thurs-Tues weekend, you use 2 days. In the military that's 4.
There will be many times where you can't take leave -- OCS, nuclear power school, prototype, SOBC, and if your boat is due to go to sea.
In your first two years of service, you can expect to be able to take leave while traveling between training commands and travelling from SOBC to the boat. This will typically amount to 1-2 weeks during each travel time.
After that, it's up to where the boat is on its deployment cycle. If you're in-port around the holidays, you'll get a week around then (typically single guys get the 2nd week where you get New Year's off). Otherwise, yea, no predictions.