On 01/09/14 I commenced upgrade of Ubuntu 12.04 LTS “precise” to Ubuntu 14.04 LTS “trusty”. (LTS is for “Long TIme Support” and it is supposed to be the reliable stream rather than experimental.)
The download and package installation took just over 6 hours.
On initial checkout, serious problems needed to be resolved in:
Working through those took about 8 hours.
My recollection was that upgrading to 12.04 LTS took four full days of work to resolve issues… so it has improved,… but a long way to go!