Infinite Time

Timers can be painfully simple, or they can send the hacker into violent tantrums. It all sort of depends on the game and system. The general idea is to just keep searching greater/less depending on the way the timer is counting, but they can vary a lot. Timers come in any size from 8-bit to 64-bit, but 16-bit and 32-bit ones are more common.
Basic Timer Hack Strategy

1. First get to the point in the game where the timer starts counting up/down. Then pause/freeze and start an unknown value search (initial dump). 32-bit is probably the way to start, if possible. If this fails, try 16-bit and so on.
2. Now let the timer count down just a little, and do a less than search. Keep letting it count down and doing less than until the results are narrowed enough to test.
3. In some cases, restarting the timer and doing a greater than followed by continuing with the above step will help.
4. Now when the results are narrowed enough to start testing, set a constant write/freeze on the address using a high value to see if the timer still counts down. If the trainer being used lists all the previous values of the timer, a little educated guessing can help with picking out the right result to try. Sometimes a timer will be as simple as the value on screen being the same value in memory, but don't bank on it.
5. Don't be surprised if none of the results work. Try a different bit size or one of the extra tips below.

Timer Tips

* clock style (00:00:00) timers tend to be 32-bit, but they can be found even if the user is searching 16-bit. One thing to be cautious of though is the amount of timer going by between searches. Let the seconds or minutes count up/down a couple digits, depending on the timer's speed.
* Don't be afraid to try searching the opposite direction when the search instructions above don't work. Sometimes a timer counting down on screen is really counting up internally. It's odd, but it happens. The timer on Starfox64 kept some hackers guessing for years because the timer on screen was stored a as 32-bit value counting down, while the real timer was a 16-bit value counting up! Just to add insult, the real timer was only a few bytes from the on-screen one.
* Beware of fake timers. A lot of timers there's both and on-screen timer and a real one. Test the code for as long as it would normally take the timer to run out to be sure it's really being stopped.
* In some cases, the right address is found, but attempts to freeze it will fail. This is because the game reads/writes the location faster than the cheat device can. The only way around this is to try hacking it with assembly. Hackers wanting to attempt this will need this address, so hang onto it.