Take how long time from Hotel to Airport

The New Bangkok Airport is 23km from Bangkok.
The traffic jam at 1:00-3:00 is not very heavy and there is highway to the airport.
I had a trip to bangkok last month, and only took 30 minutes to the new airport from Bangkok (by private car).
If you travel alone and have no baggage needed to 寄艙，you can check-in upto 45 minutes before the frieght departure time. So I think you leave between 1:00 to 1:30 is ok.
Of course, to play safe, early is always better.

I think if you want to play safe , you leave at 1 PM is not enough , as I afraid the traffic jam in bankok, so 1230 is better , have a little bit more time