I'm running Windows 7 x64 on a fast machine with 8GB ram and it takes 4+ minutes to go to sleep. Is this likely just because it's writing 8GB RAM to disk or is there somthing awry?
It can be a factor of how much memory you have but is also highly related to what you actually have running. On average my 4GB x64 box goes to sleep in between 30 seconds and a minute, but if I have VMware Workstation open (and busy) it can take around 10 minutes to fully sleep.
If it is actually sleeping and waking without any problems, I would not worry about this.