So, this is from the Avengers page, but it is the same set of rules I've seen the last two GSC. My sister deferred PHM 2014, and I considered deferring this year but didn't. This should give you an idea of the rules. I can't recall how long it took before it opened up, but I'm guessing it will be maybe 1-2 months before? I checked and it's not there for WDW marathon weekend yet.
As far as I recall, you must defer to the same weekend. And you don't get a refund, your fees (minus the processing fee) are applied to the race you defer to.
From the runDisney.com website, under the registration tab for Avengers weekend:
Deferral Policy & Request Process
- Deferral requests are on a first-come, first-served basis and are subject to capacity limits.
- Once deferral capacities are full, no further deferrals can be taken and no exceptions can be made.
- Deferrals are not available for the Avengers Super Heroes 5K or runDisney Kids Races, or any other race weekend purchases.
- Ancillary Events and/or Commemorative Items cannot be refunded if you Defer.
- If you are currently registered for the Avengers Super Heroes Half Marathon, Infinity Gauntlet Challenge, or Captain America 10K in 2015 you may apply to defer your registration. Deferral requests will close on November 11, 2015, or sooner depending on deferral capacity, as some categories may fill sooner than others.
- Deferral requests will be assessed a $55 processing fee.
- Request a Deferral.