Charge controller settings

It may be that you need to plan for replacement of the bank, but there are a couple of things worth checking first.

The way the charge cycle should work, is the controller wakes up with the sun, and goes into bulk mode. It will use whatever power is available to charge until voltage reaches the absorb voltage setting (generally around 80-85% SOC). This may take something like 1 hour for each 10% of capacity below ~80% in full sun (so a bank at 50%SOC might take ~3hrs). When it hits set absorb voltage, the absorb timer starts, and the voltage is held at the setpoint until the time expires. The controller then lets the voltage fall to the float setpoint, and holds it there. Generally, a timer on float isn't needed with solar, as it's fine to stay in float until the sun sets.

From your description, a couple of possibilities come to mind. It's possible the absorb and maybe float voltages were set too low, so the absorb voltage is hit early. Assuming the batteries are flooded, absorb voltage should be ~29v (temp compensated).

The other possibility is the bank is failing. One symptom of a failing bank is it will appear to charge fast, but will also discharge fast (effective capacity has dropped to unusable).

As a first step, I'd suggest checking the controller absorb voltage setting, and also checking the actual voltage at the charge controller battery output terminals, and at the battery side connections with a multimeter when the controller is in absorb.