The short is that it shouldn't be an issue.

Lithium Ion batteries have a very low self-discharge that is negligible.  They wouldn't have any discharge if were not for the onboard protection circuitry.  It sounds like you may be speaking about the Nickel-based issues where the capacitance curve was constantly diminishing with the use of and the life of the battery.  With Lithium Ion and the fact that the drill is micro-processer controlled the capacitance curve is not constantly diminishing, but rather has a more sudden and steep drop off. 

There is no problem with the Lithium Ion batteries and the TRC 3 charger to leave the battery on the charger, but there is no strong benefit after the battery has been fully charged either.

The best piece of advice to all Festool T+3 users is to just use them.  There are multiple layers of protection and control with sophisticated electronics.  And the drill, charger and batteries have a 3 year warranty.

Here's some general information for prolonging battery life:

• Lithium-ion batteries should be charged early and often. Frequent use is beneficial to the performance and lifetime of the battery.
• Li-ion batteries should never be depleted to below their minimum voltage so we designed the onboard electronics prevent this from happening.
• Lithium-ion batteries do not need to be fully discharged before being recharged. Charging is possible at any time without concern for the memory effect.
  
• Li-ion batteries should be kept cool. The high temperatures found in cars cause lithium-ion batteries to degrade rapidly. Please be mindful of where you place the batteries in your vehicles and remove them from the vehicle if being stored for a prolonged period of time.
• Store batteries in a cool, dry place. Protect the battery from moisture, water and heat. Remove them from vehicles if being stored for a prolonged period of time.
• Storing the battery in an energized (plugged in) charger will keep the battery charged and ready. (leaving a battery on an unplugged charger may dissipate the battery and could damage the cells)
• Store the battery in its packaging (Systainer) to help reduce accidental short circuits or discharge. (shorting out the contacts of the battery)

Temperature ranges:
Charging = 5 - 45º Celsius (41 - 113º Fahrenheit)
Storage = 0 - 25º Celsius (32 - 77º Fahrenheit)
Battery Protection: Onboard circuit shuts down battery over 80º Celsius (176º Fahrenheit)

If you're asking about the batteries' ability to retain capacitance over time if not in use, there should not be an issue with it dissipating.  That was answered by the statement:

"Lithium Ion batteries have a very low self-discharge that is negligible.  They wouldn't have any discharge if were not for the onboard protection circuitry."

In simple terms, there is very little change in the capacitance over time if the battery is not in use.  From personal experience, I have a battery that sat unused for 6 months or so.  I put it on my T15 and it worked fine as if it was full charge. 

If others have any experience on the subject with the new Lithium Ion batteries, please share.

I use my t15 off and on at work and one charge will last me on average 3-4 weeks.

I have a bosch and hitachi li-ion drills for use at home and at times they would go weeks without any use but still have plenty of power when I do come to use them.

Hope that helps in your enquiry.