Rough Battery (9V Alkaline) Level Estimation with Analogue Reading
Expect 9V as a fresh battery, and about 6-7V as a dead battery (I guess you need 5V in your device, right?)

In case of a good battery, continous load shall not give you really different measurements. Of course, the battery voltage may drop by say, 0.1V, but it is really something you can ignore.

You may want to take into account the discharge curve of your battery to improve accuracy of power estimation: http://www.powerstream.com/9V-Alkaline-tests.htm This could be done by having a simple array converting measured voltage to expected % of remaining power.

I suggest you to code some histeresis, otherwise you will see your status bouncing between "full" and "half" states when the battery is nearly drained to half.

I agree with the other comments that a 9V battery has only one advantage: convenience. It is really not the best power source for your circuit, you may run your stuff from 3xD cells (4.5V connected right to the 5V rail, bypassing the LDO) for about 40-50 times longer than from a 9V battery.