One way to approach it is to break it down from days to hours, and keep track of the hours achieved over the course of his adventuring time.
I would insist that the first 50 hours of every quarter be spent with a tutor, while the remainder can be done on the road, alone time study or simply practicing with other people.
Example: Grubbins the Half Orc is trying to learn to speak Elven. Overall it will take him 1000 hours of study to do so. He finds a tutor and spends a week learning the basics of the language, purchasing a nice book to help him learn.
Grubbins' group then set off to track down a Dragon, wreaking havoc on the local countryside. Each day they spend 8 hours marching, a couple of hours hunting for food and 2 hours taking short rests. With 8 hours of sleep that leaves Grubbins 4 hours a day to study Elven. Because he is active, he rolls an Intelligence check DC 10 to ensure he can benefit from the day's session. He rolls a 12 and manages to retain the knowledge he read, adding 4 hours to his total time spent learning.
Amrien, the party's Elven Wizard, was an archivist and is a trained linguist. She offers to assist Grubbins learning Elven. Thanks to her assistance, she is able to double the benefit of the time spent studying. Therefore when Amrien assists, Grubbins can benefit from 8 hours for every 4 he spends.
Over the next fortnight they find, engage and slaughter the poor dragon. The next 10 days they spend returning for their reward. Grubbins succeds on 14 of the 24 Intelligence checks, with Amrien capable of assisting 10 of those, for a total of 100 hours towards learning a language (in the month he has spent on the road, including the initial day for those concerned the maths doesnt add up!)