As pointed out by others, the probability of success is defined on p.B171. That's all you need to know to do the math.
As to what math:
To determine the number of successes achieved over a specific time period/specific number of rolls: multiply the number of rolls by the probability of success of your effective skills/attribute to determine how many rolls will be successful, and drop fractions.
Example (same as provided by a previous post): 30 days of healing means 30 rolls. If your effective HT for healing is 11 (62.5%), that would give you (30 x 0.625 = 18.75, dropping fractions gives) 18 successes, so you would regain 18 HP over 30 days.
To determine how many rolls/how much time is required to achieve a certain number of successes: divide the number of successful rolls required by the probability of success, then round up.
Example: You need to recover 12 HP, thus make 12 daily healing rolls. Your effective HT for healing is 11 (62.5%), you would need to make (12 / 0.625 = 19.2, rounded up to) 20 healing rolls, or it takes 20 days to fully recover.