It depends on the country, the company, the drug and the product life cycle of the drug and the size of the territory you are looking after. A big part of the MSL role is externally facing, meaning interacting with external experts and therefore you will have to travel to meet these experts. How far and for how long you have to travel can really vary. Some positions require to fly and to spend nights away from home whereas other positions require primarily driving. In this case, MSLs are almost never away from home.
Some MSL's job description mention 80% travel, however none of our FSTP alumini working as MSL have never traveled 4/5 days per week. Furthermore, if after a year you feel it is too much travel, you just move to a different MSL role or perhaps a more office based job within your company, as you are now an experienced MSL and you are worth your weight in gold!