This is one of those "there is no right answer" questions. The issue isn't so much how often, but how well the mobile employee and his/her manager, co-workers, or customers agree on what that frequency should be. If the employee thinks that once every four hours is enough, and the boss thinks that once every hour is enough, they will be headed for problems. There's no objective way to say if once every hour vs. four hours is correct, but when there is no discussion/negotiation about desired frequency, problems will occur.

One factor to consider is how easy it will be for the mobile worker to actually connect - unless you're using cellular modems, the employee will have to find somewhere to plug in a phone jack and get the mail. Depending on how difficult that is, and how much time it takes away from whatever the employee is supposed to be doing (e.g., selling, service, etc.) it may meet the goal of checking mail often but at the expense of customer service.

Some email systems have the option of coding messages for urgency, and the triggering a pager - the same as some voice mail systems have. An ideal situation might be to have the mobile worker check mail once every 4 hours (for example) but if he/she gets a page saying there's an urgent message waiting, that much be retrieved within 15 or 30 minutes.

Last, you need to think about the difference between day and night, weekday and weekend, work time and vacations/holidays. The expected connection frequency should probably apply to agreed-upon work hours (e.g., 7 am to 7 pm weekdays) and there might be less-frequent connection expectations for those other periods, going all the way down (ideally) to no connection while on vacation.