Respawn times are usually between 3 and 7 seconds...considering it takes a fraction of a second to choose a weapon, you should have plenty of time to choose your weapon before you even spawn. Honestly, if you can't pick your weapon fast enough, you deserve to die. I suggest memorizing which number key corresponds to which weapon; choosing with the mouse takes slightly longer. Also, if you just click on a random area on the screen, you will choose the last weapon that you chose instantly.