Thanks Paid. Forgot to check the manual that came with radio and it says 5/7 hours. It says when batteries are exhausted a red light will show but it does not appear to say how long the batteries will last before recharging again.
If you're using them in an internet radio that will also charge them when plugged in, then you can expect subsequent charges to take up to a similar length of time to the first one depending on their state of charge when plugged in.