Distance from bulbs is generally 18-20 inches.
Regarding frequency of use, there is no one answer for everyone. Frequency can depend on a number of factors; how you feel, if you have certain health conditions, etc. It is best to start off very slow, 3-5 minutes per day and increase time slowly. The benefits of near-infrared (NIR) color light therapy are accumulative, using 5-7 days weekly for 15-20 minutes would be much better than 30-45 minutes once or twice a week. For certain conditions or goals, using more than once daily can also be very beneficial.