There is no limit to how many times you can spray a spray, though there is a cooldown in between sprays. Sprays last until you spray elsewhere, which means you cannot tile a wall with your spray. Furthermore, some servers have it set so that no one can spray.
Consult your options panel for the spray keybinding. The default is bound to "T".
Installing a spray is kind of a hassle if you've never done it before, check out this page: http://tf2.gamebanana.com/tuts/4488
- Find your image, it must be square the following sizes are acceptable: 64*64 128*128 256*256.
- Download VTFedit.
- With VTFedit, Import your own .JPG,.BMP, etc... and convert it to a .VTF file (Valve Texture File)
- Now place the .VTF file in: (your dir)\Steam\steamapps(your username)\team fortress 2\tf\materials\VGUI\logos
- To fix the quick view appearing blank copy the same .VTF also in: (your dir)\Steam\steamapps(your username)\team fortress 2\tf\materials\VGUI\logos\UI
- In game, go to the options and click import the spray and browse for it in the \logos folder (not the UI folder!) and validate. (this will create a VMT file near your VTF)