If you can accept a 19 hour recharge rate things get a lot easier.
Normal rule of thumb for 0.1 C charge rate on NiCads is about 14-16 hours recharge time which is 1.4X to 1.6X the mAH output to 'C' rate mAH's for charging.
Most NiCad's will tolerate up to 0.1C continuous, unterminated charge without too much degradation. If you applied a constant current of about 0.22A (about 0.08C rate) into a 2.8 AH NiCad you should be close to what you need.
I doubt that there is any real way to to put a real number to that, since the change in capacity over time is dependent on many things, including:
- number of charge/discharge cycles
- depth of discharge,
- ambient temperature
- charge rate
- discharge rate