Training

When a character has earned enough experience to attain the next level, immediate advancement is generally not allowed. The character must receive training to advance. When training, a character studies his skills under a tutor, taking the raw knowledge he has gained and honing it into measurable improvement. On the average, this takes a few weeks (depending on the tutor's ability). and it is normally done during the characters non-adventuring time.
A DM can also rule that the circumstances are not appropriate for the character to advance in level. For example, the game session ends with the characters deep in an abandoned mine complex. The party has just finished a battle with a band of gnolls and faces more such encounters before it can reach the surface. The DM rules that the characters receive no experience until they leave the mines, because he doesn't want them to increase in level in the middle of the adventure. He is perfectly justified in doing this. And if the characters live through the adventure, they will undoubtedly profit from it. either in experience points or knowledge gained.
Advancement in level does not occur until training has been completed, although accumulation of Experience Points (XP) will occur until the character accumulates enough Experience to take them to another level hugher, e.g., if a 2nd-level character accumulates enough Experience that would take them to 4th-level, the character stops accumulating Experience points at 1 point shy of the upper level. Thus, a 2nd-level fighter would stop accumulating Experience at 7,999 XP, 1 point shy of 4th level.
Training costs 100 gp per level to be attained (300 gp at 3rd-level) and take 1 week per level (3 weeks to train for 3rd-level).
Training continues through 9th-level. Once 10th level is attained, training is no longer needed.