In the guidelines for outside counsel that I have reviewed, law departments are all over the place on billable travel time.
Some let outside lawyers bill time only if they travel during business hours. Others let them bill only while working on their matters, even though that “guideline” is completely unenforceable. A third possibility lets lawyers who travel bill two hours (or some number of hours) to a matter – simple, and perhaps a balancing of the equities.
A more lax view allows clients to bill for travel time at half of their hourly rate or some other percentage. Yet another variation is that only one person may bill for their time on whatever the arrangement is (this is akin to the notion that only one person at an internal meeting can bill for the time of the meeting).
I can imagine a policy that allows time to be billed on longer flights but not on short flights. Another twist has to do with whether the client insists on the travel or whether it is the law firm’s decision as to when and where it should do something.