I managed 800+ PCs and laptops in a school running Windows XP then Windows 7 via WSUS & we regularly had problems like this which were solved by neil_g's suggestion of clearing out the software download folder & restarting services.as said, clear out the softwaredownload folder (google) and try again.
if your internet isnt amazing or has dropped then it could be a corrupt update/file.
it certainly shouldn't be a common issue (at least ive never had it on 150mb VM).
its normally 30 days