You could try downloading the Twixtor demo. Then slow down the clip to 50%, precomp it, slow it down to 50% again, and so on .. you’d have to do that 4-5 times to get it to eight minutes. You might have to render it each time — I don’t know if your system could handle the precomping.
If you like it, buy Twixtor.
I suggest the stepped 50% approach to give Twixtor something to work with, since going from :20 to 8:00 would be too much.