2 - 4 horas
The Mac mini is a really capable little machine for which I decided to sell my 2010 Mac Pro for. Upgrading with a dual SSD in RAID 0 gives it a real kick which makes it even more competitive in the graphics / design era.
If you've some knowledge on how a computer is built up, what are its main components, than you won't have any problems - especially as the iFixit guides are really walking you through the process.
Take really good care to insert all connections back into their place firmly as I almost fried my mini with the CPU fan hanging loose on one end of the connector.