very simple.

on top of your gamemode put.

forward Payday();

now in OnGameModeInit() Put.

SetTimer("Payday",600000,1); // 600000 = 10 Minutes !

Now you need to create a public.

public Payday()
{
for(new i=0; i<=MAX_PLAYERS;i++)GivePlayerMoney(i, 1000);
SendClientMessageToAll(COLOR_GREEN,"PayDay: 1000$ :D");
}

if you want to know exactly how many seconds, minutes, hours or even months you can download the SetTimer Converter: http://www.4shared.com/file/AppbWJ30/Converter.html