5 - 15 minutes
My mac mini was a bit sluggish with OS X Mavericks and 4GB of ram. I don't know why. I really don't multitask much. I guess the OS is tuned for more ram. After my upgrade things were noticeably snappier.
The Mac mini has user serviceable RAM, so it was quick and easy.
Vote with you wallet. Buy the macs that have user serviceable ram.