Training to Gain Levels

Downtime Activity: Training to Gain Levels DMG p131 As a variant rule, you can require characters to spend downtime training or studying before they gain the benefits of a new level. If you choose this option, once a character has earned enough experience points to attain a new level, he or she must train for a number of days before gaining any class features associated with the new level.
  The training time required depends on the level to be gained, as shown on the Training to Gain Levels table. The training cost is for the total training time.
  Training to Gain Levels
Level Attained Training Time Training Cost
2nd-4th 10 days 20 gp
5th-10th 20 days 40 gp
11th-16th 30 days 60 gp
17th-20th 40 days 80 gp
Source: DMG, page 131