Most people are referring to the number of business days traveling or in another city. 30% would be 3 days out of every 2 weeks. You might fly out to visit a client on Sunday, work there Monday through Wednesday, fly home Wednesday night, then work locally the rest of that week and the following week.
The IRS considers business mileage as any driving done solely for business purposes. For example, driving out of the office to meet clients, get supplies, or run other business errands. Note that commuting between your home and your place of work is not considered business mileage.
So if you work 50 weeks a year and have to take 5 business trips a year each of which lasts 1 week (say, visiting a client as part of a project), that would be 10%.