Which one is better?
a) Use all (or most) of your free days in a row. You totally disconnect from work issues, but when you come back your’ll have a flood in your mail inbox.
b) Use them little by little, so do small breaks during the year. You don’t totally forget about work things, but you recover some energy several times.

Basically the thing depends on how many days do you need to reach a real disconnect status. And this also depends on what are you going to do during holidays: if your holidays include a lot of activities, you don’t need a lot of time to remove any track of work from your mind, but if you’re going to spend your time at home or next to your office, you’ll sure need more days.

Then, the key is to find the average amount of days you need to disconnect, and do as many breaks as you can with this length… if your employee lets you so.