After allowing the Drano to work on the clog for 15 to 20 minutes or even for an hour you can then get some boiling hot water.
Then pour the boiling hot water down the drain until the clog is cleared out.
I would recommend pouring down at least a gallon of boiling hot water to flush the Drano out and then next allow the hot water from your faucet to run for around 4 to 5 minutes to help flush out the clog.
5 or 10 minutes of the hot water running should be more than enough time and hot water to flush the Drano and clog clear out of the system.
If the clog is still there then the clog could be further down the drain line and then you might need to snake the drain to clear the clog.