A question arose in our group about the retraining rules from Ultimate Campaign.
Retraining takes all your attention for 8 hours per day for a number of days based on what you're retraining. You can't perform any other strenuous activities while retraining, such as marching, adventuring, or crafting magic items. You can retrain only one thing at a time; you must complete or abandon a particular training goal before starting another one. Abandoning unfinished training means you lose all progress toward that training's goal and all costs associated with that training.
The question here is: what if you want to train twice in one day? Can you spend 16 hours in 2 consecutive sessions to decrease the number of days you have to spend training? It seems like the rules lean against allowing that, but there's nothing forbidding it either. The character in question also has a Ring of Sustenance, so he can rest between every session if that is relevant.
Like crafting, traveling, and pretty much everything else, you are assumed by default to only spend 8 hours per day doing extended activities. Some provide rules for allowing a character to spend extra time, and some don't.
For retraining, I would rule that exceeding the usual 8 hour limit would cause the character and their trainer to be affected per the forced march rules, as it seems the closest analogue given by the rules. If they went to 16 hours, I'd ask when they had time to do things like eat, drink, excrete, bathe (if they're into that sort of thing), and so on; if any couldn't be accounted for, then I'd ask if they'd rather sleep in or start taking the usual fatigue penalties for lack of sleep.
The ring of sustenance shortens the required amount of sleep and takes care of some bodily needs. Whether this means that the player is now on an 18 hour cycle or that the player can spend those extra six hours doing useful things is up to the GM. The ring does nothing to alleviate the effects of sustained (heh) effort, though. With two sleeps a day and either rotating trainers or a second ring, I'd say you could do two 8-hour sessions, though you've consumed half of your downtime that way.
A GM could certainly rule that you cannot double up on retraining. Possible justsification might include that it takes time and rest for the mind and body to assimilate information and heal microinjuries gained during the day and that prolonged training hits a wall--either overstress the body or cognitive overload. In that case, you're better off spending that downtime doing something else, like crafting, working, investigating, or traveling.
And that would be one of those GMs I mentioned. : )