There is no standard time for Running Bars. The facilitator will follow the energy and deliver only what you are happy to receive.
The average session, where all the points are run, is about 90 minutes however Bars can be done in as little as 20 minutes if you only want to work on one or two areas.
Children run faster as they usually have less resistance to receiving than adults.
There is no such thing as too long of a Bars session. For that matter, people have had results with only 5 minutes of having a single Bar run. Longer sessions such as 3 to 6 hours can often be dynamic, with life changing results. However having your Bars run more often, rather than for a longer period of time can be beneficial.
A facilitator will always ask you what works for you. It is totally up to you, and there is no wrong way to have a Bars session.