Ultimately i care more about DOMA, but I think it will be 10-15 years, when SC takes the case again (as senseless as that is, not like the merits change in that time). Getting 60 votes on anything is rare and the repubs in the senate have proven they are anti-gay at all costs. This may change if they lose another election and today's seniors die off/become senile, but not before. The immigration bill fiasco and their brief to the court on this very issue should make that clear.
A state-by-state will take longest of all. Can you imagine Alabama rednecks voting for this before 2050? But even that situation is quickly becoming more tolerable (3 states in a month?), once DOMA is defeated. So crossing my fingers for next month, but i think 10-15 years, then it will be forced on Alabama rednecks, probably thru SC but conceivably the legislature by then.
I believe that it is highly likely that DOMA will go down this summer. I think that even if there weren't enough Justices there for the equal protection clause to take it down, that some of the Justices would be willing to sacrifice it for the states' rights ruling. Basically sacrifice DOMA so that it protects/takes longer to strike down state bans. I think, like others, the Prop 8 case will be a narrow ruling applying only to California or those that would grant marriage then take it away.
Now, I think we will see a few more this year legalize ssm then it will slow down again as states work to vote away the constitutional restrictions. Then, we may see another increase. It will continue this way until one of a few things occur. Either the SCOTUS finally takes up a state case to strike down the laws that isn't as easy to make a limited ruling on (a military couple would be good for this since they have to move to states that may not recognize them as married, plus the new rules in place treat same sex couples and opposite sex couples differently because of the very fact that same sex couples cannot get married in all states)(another possible good case for this would be if some idiot actually did tell a transgender MTF and her wife or transgender FTM and his husband (less likely couple to begin with though) that they cannot be legally recognized as married eventhough they were married prior to the change, as an opposite sex couple). Or the US Congress passes an opposite act/bill than DOMA that tells states they must follow the Full Faith and Credit Clause of the 14th when it comes to marriage. Otherwise, it will take many years, possibly a couple more decades, to get all the states to finally overturn their same sex marriage bans because there are unfortunately a lot of stubborn people out there.
"A woman is like a teabag, you never know how strong she is until she gets in hot water." - Eleanor Roosevelt
Keep your religion out of other people's marriages.