Training Bees have a Mission Cap which determines how many missions they are able to go on. 

You can add more Missions to a Bee's Mission Cap by upgrading any stat. Each upgrade will add 10 additional Missions to that Bee's Mission Cap. Visit the Bee Lab to upgrade your Bees.

You can keep adding to your Mission Cap until your Training Bee reaches his Mission Span. Mission Span is the amount of days the Bee can perform Missions. Mission Span cannot be extended.

Once your Bee reaches his Mission Span, you can convert him to shards. Visit the My Bees section to convert your Bee.

Retired Training Bees can be converted into shards. This will remove the Bee from your hive.

You will receive 5 Generations Bee Egg shards when converting a Training Bee that has completed at least 60 missions (must be a Certified Beekeeper or have purchased a Season Pass this month to qualify)

You will receive 40 shards for each Training Bee you convert. The 40 shards are randomly chosen and can be any of the following: Shield, Poison Bomb, Cursed Honey Drop, or Sonar Buzz.

NOTE: Retired Training Bees can still be used to do Quests and PvP. 

For more information, see this link