A PP3 alkaline battery (most know it as a common 9V) will have a terminal voltage very close to 9V when new. If you draw 10mA for a while, its voltage will fall to about 8.4V. As it goes dead (flat) it will begin falling more rapidly. If you draw 100mA it will fall more quickly and most brands will be dead between 1 and 2 hours.
P3 batteries are also available in NiCd, NiMH, and Lithium. Their actual voltage depends on their construction. (Some NiCd and NiMH have 6 cells = 7.2V, others have 7 cells and are 8.4V).
Do not use the information as a specification, because I didn't describe any particular battery. Manufacturers have data sheets that give detail about each product. Duracell and Eveready might be good places to look.