PhotoPass photos only live on your account for 45 days from when they are taken. There is a one-time 15 day extension that can be purchased if you need to extend that.
Memory Maker, when purchased at the advanced discount price, cannot be used to download photos for 3 days. Any pictures taken before the purchase, or within that first 3 days after the purchase, are not eligible for Memory Maker. (If you pay full price for MM, you have access to photos going 30 days prior to purchase, plus the download window.)
Memory Maker does not become "activated" until you download a photo or group of photos. Once you do so, you have 30 days to download whatever photos you want.
So technically, you could purchase Memory Maker at least 3 days before the first photo is taken, wait until the last possible day to download that first photo, and then have 30 more days to take more pictures and download them too.
That's 2 1/2 months of photos, (3 months if you purchase the 15 day extension)
So for multiple trips, it depends on how far apart they are.