If you have a .edu email address, you can get Prime for half price for up to 5 (I think ) years. So that's helpful.
It's up to four years. I know because I have a student account (due to the fact I'm still a student) and noticed it tells me I get to have the student price no more than four years from now.
After reading the first page of this thread, I thought I'd check out Prime Pantry. Didn't see anything worth my while. DH then managed to break his phone (iPhone 6) last Friday that we've only had for 4-5 months so we're still 19-20 months away from paying it off (his might be four months as it was a Christmas present from me; I've had my 6+ for five months). Anyway, I needed to get him a new case for his replacement iPhone as quickly as possible so I figured $49/year would be worth it for the 2-day shipping and to be able to watch the latest episodes of "Mad Men." Well, the episodes cost extra outside of the Prime membership so that was one strike. However, you can watch the latest episodes of "Mr. Selfridge" and "Wolf Hall." Okay, so I checked and even with a season pass for both "Mr. Selfridge" and "Wolf Hall" *and* paying for two-day shipping by itself, I still would've spent $10 less than for that Prime membership. I canceled it a few minutes ago.
For someone like me, it's not worth having so I may have unlimited two-day shipping. It is a rarity when I need something that quickly because, busy as I am, I spend a ton of time online so I'm usually already in place to purchase things far enough in advance not to need faster shipping. Our house is well-stocked enough that there's nothing I need to order through Amazon I can't get much easier and faster by going to the grocery stores that are pretty close to our house (we drive by one every day to/from work). In the case of the cell phone cases, I needed those as fast as possible without having to search stores finding the specific ones I wanted and two-day shipping was the fastest option online with Amazon (DH was the one to break his phone so I think he's the one who should have to physically call around and go to stores to replace the case, but I was willing to order them online to protect his phone faster -- the screen protector and case I had for his first phone actually did protect his phone for a few months because he was constantly dropping it; he finally dropped it once too many times after he'd already done serious damage to the screen protector and shattered his iPhone screen). I learned and ordered an anti-slip impact-absorbing case which arrived one day after the phone arrived (so I prayed that he wouldn't drop the phone again in that 24-hour period).
Anyway, I was really hoping that Prime would turn out to be a great benefit for us, but it really isn't (we have Netflix and Hulu so having access to yet another company's video library isn't worth it). We also already have a lot of stuff so I'll typically save for later things I'd like to obtain (because they're usually not things we need -- just things I want to have) and when I'm buying stuff through Amazon, I'll check my list and see if there's anything I want to add in order to either a) qualify for free shipping or b) not have to pay for shipping since the item I'm ordering initially already has free shipping covered.