Do the lights now come on immediately and stay on after the car sits?
Things that can be assumed:
#1 - If the ABS light is on, you have no ABS brakes. That light only comes on if the ABS computer has disabled itself.
#2 - If the ABS light is on, and you have good brake assist, the ABS Pump relay, and the ABS Pump are working.
#3 - If the Fluid level is fine, and you don't notice the ABS pump cycling often, it probably isn't the Accumulator
Possible problem - A wheel speed sensor has failed.
If the ABS computer does not get inputs from all 4 sensors, it will disable itself.
The sensor could have come loose and is now too far from the sensing ring. The sensor's wires could have become damaged, or the sensor could just have failed. They are Hall Effect type sensors and can fail.