While it is possible to become a Field Manager with a high school diploma, the vast majority will be educated to Bachelor's Honors degree level in Business Management or similar. Some will take their education one step further with a Masters.
In addition to academic qualifications, the role of a Field Manager is one that requires industry-specific training and a wealth of experience. It is common for Field Managers to have worked their way up the ladder from Field Engineer or lower-ranked supervisory roles. There is no set time frame on how long you should have been in those roles before progressing to this senior position, but it's usually at least a few years.