One of the first questions people often ask when approaching the practice of meditation is "How long should I meditate?"
The Tibetan word thun (pronounced toon) means practice session, but it can also mean a prescribed dosage of medicine. When you think about taking medicine the context for how long you should meditate starts to make more sense.
A traditional thun is a three hour practice session. The Dalai Lama is quite well known for his daily practice, which consists of rising early and meditating for several hours before most of us even get out bed. Though we should aspire to such a practice, there are numerous variations available.
The short practice session. This is a session that lasts less than three hours. It could be 15 minutes or even an hour. I can tell you from my own experience and from numerous others that it really takes 15-20 minutes for real stability to develop, so at a minimum 20-30 minutes a day would be a good baseline practice while you build up the endurance to practice a traditional thun. Doing two sessions a day can help you add time on the cushion but also fit into a busier schedule more easily. You could call this your maintenance dose.
The day-long session. The day long session, or nyin thun, is an intensive bout of practice which allows for greater depth and a wider range of experience. A full day session consists of what is commonly called the four session yoga, traditionally from 5-8am, 9-12pm, 2-5pm and 6-9pm. That is a pretty intense day. Once you have a couple of these days under your belt they won't be as intimidating. This is kind of like a bolus dose, meant to quickly get you to a therapeutic level.
The practice intensive. The practice intensive, or sgrub mchod (pronounced drubchod) is any time period between one and seven days, typically two to three days is customary. These practice intensives usually have a strict schedule for each session of the day and are quite effective for developing greater depth and clarity than the shorter practice sessions. These sessions are kind of like receiving a high dose of a corticosteroid for a short period of time, very focused and specific.
The great accomplishment. The great accomplishment, or sgrub chen (pronounced drubchen) is a seven day practice intensive. These practices require a strong intention and dedication to carry you through. It is very common for people to experience a lot of resistance around day three and how they deal with that resistance determines the remainder of the practice. Day three can often mark a breakthrough or a breakdown. Having completed a drubchen it is much more natural to try to continue your daily practice in the traditional thun format, since you are no longer limited by the conceptual apprehension of sitting for three hours a day (because you just sat for 12 hours each day the past week). These sessions are similar to receiving a course of antibiotics to rid yourself of infection.
Once you get past the drubchen, there are longer length retreats that you can pursue. Traditional time frames include a one (lunar) month, a three month, yearlong and a three year and three month retreat. Each of those would continue with the standard four session yoga format, with guidance and instruction required prior to entry.
All of these are for the purpose of transforming your mind. Any of them will do that, but probably the most beneficial and the one that provides the most return is your daily practice. Build a strong daily practice and fit in the short bursts of intensity over time.