When using a phrase such as the above, it's always a plural possessive if you're talking about more than one year's/month's/day's anything. ie. In 10 years' time, more than four months' therapy, less than one day's pay. Plural things are plural, even when they have an s already. That's where it gets confusing, I admit.
I won't even go into the above usage of over instead of more than. I hate over when used like it is up there, but apparently, some people are becoming more accepting of it. But remember, kids: just because some say it's OK doesn't make it right.
