It is customary to check tefilin twice in each seven year period. Tefilin do not usually require checking, but you should periodically check:

• Tefilin of the type that can become pasul (due to white-washed parchment).
• Tefilin that are moved a lot, such as from place to place where there are large changes in temperature.
• Tefilin in humid climates, such as Florida.

NOTE Ask a sofer for advice about any of these cases.