Reduction in frequency:
If you reduce training days by 1/3 or 2/3 (that is, from six training days per week to four or two days per week) and maintain the training intensity and duration (work as hard and as long as before), you can maintain your endurance.
Reduction in duration:
If you reduce the minutes per session by 1/3 or 2/3 (or from 40 minutes/session to 26 or 13 minutes per session) and maintain the training frequency and training intensity (work as hard and as often), you can maintain your endurance.
Reduction in intensity:
If you reduce training intensity by 1/3 or 2/3 and maintain the training frequency and duration (work as frequently and as long), there are significant losses of endurance fitness.
Training frequency and duration can be reduced with little effect on overall endurance. However, when you train, you need to train at a training intensity similar to what they trained at during the season. The quickest way to lose endurance is to reduce training intensity.
Other Offseason Considerations:
During the off-season, if you reduce training volume (as days per week and/or minutes per day), you will be reducing the number of calories burned during exercise. To maintain weight during a period of reduced training, you may need to reduce your food intake.
There are some players who may need to lose weight to improve their performance.
First, don't make this decision without some sound advice on whether weight loss is desired and get advice on nutrition and weight loss goals. Once this decision has been made, the season for weight loss is the offseason, not in-season.
Trying to lose weight during the season is a quick way to poor performance and possible injury. Save weight loss for the offseason.
Most athletes can be better in their sport if they are stronger. Strength training does some things, but not others.
For example, the stronger player will be able to resist physical challenges better and be more resistant to injury. However, strength training is not real effective at adding distance to your goal kick or power to your shot.
The off-season is the best time to improve strength and power. While the details of weight training for soccer is an entirely separate article, the soccer player should select activities that improve their overall strength and not focus exclusively on their legs on the notion that they will improve their shooting. To be a better shooter, go out and shoot.
Once the season begins, the goal of strength improvement gives way to the goal of strength maintenance.
There is a genuine concern among the soccer community that players compete in too many games each year.
Games for school teams, club teams, in and out of season tournaments can mount up to the point where the only rest a player gets is when they get injured. There needs to be planned periods of rest followed by a planned re-establishment of fitness for the next season.
Rest is important, so take some time off play Ultimate Frisbee, roller-blade, cycle, hike or any number of other activities by being active, but be away from soccer. Both your body and you mind need the rest.