I have a very good heater that I used for Blu's one gal tank before he got moved into a very nice 5 gal. I'm not really sure how to work it anymore, since the dial was always mildly tricky and confusing (turn it towards the - one day, it gets hotter, turn it towards the + the next and it gets even hotter)

Since I'm not really sure what the actual setting is on it, how long should I wait for the water to warm up until I put my new betta baby in there?

He's in his cup right now and is getting 25% water changes every 90 mins or so from the water in the tank he'll be moved into. The water they had him in was filthy, thus why so many changes so often... I want him to be healthy but don't want to shock him. He seems very young...

I was actually going to ask you the wattage on your heater and do an actual physics calculation to answer this (WHAT IS WRONG WITH ME?)

In a er...more practical fashion, I think getting a thermometer would be your best bet. It also depends on the temperature of the water you put in initially, the size of the tank, and the strength of the heater.

If you DO have a thermometer however, then all you can do is wait and just float his cup in the tank. From a physics point of view, the heat transfer from the heater to the surrounding water to the plastic cup to his current water should be slow enough not to shock him.

Floating his cup is more than likely what I'll end up doing, but what I'm worried about is it overnight getting way too high and boiling him. D: Should I just wait until tomorrow night to see what the water temp tops out at before floating his cup?

Again it depends on the strength of the heater, since you used it in the 1 Gal, I can't imagine that it would "boil" a 5 Gal tank... I'd be more concerned as to it overloading and breaking the heater because it would be on most of the time to keep up with heat exchange.

Just the fact that the newer tank is considerably larger than the previous one, I don't think overheating should be a problem, if it were a problem, it would have overheated the smaller tank rather than the 5 gal.

=( It's still in the smaller tank. I got a 5gal-10gal heater for the bigger one so this one has been unused for a few months and that's the problem. I don't know the settings anymore for the 1gal heater.

edit:
I have a new fish. Blu's tank (the 5 gal) is all set up and he's happy. So the 1gal got stored away with it's mini heater for a bit-- Then I got Prince, my new Betta baby and busted out the 1gal to let him grow up a bit and keep an easier eye on him until he's grown up to a bigger tank.

If it is a proper heater, it should not take long to heat up the water... my 50 Watt heats up my 5 Gallon and stabilizes after about 30 minutes? Maybe even less, I don't really notice because I float the cup in there anyway to keep Bobo warm during water changes.

If it is underpowered, it would take a very long time and it may be very unstable...dropping in temperature every few hours because the heater cannot handle it. It also might damage the heater

I don't have any predictions on overpowered heaters, but my guess would be also wildly fluctuating temperatures and damage to the heater constantly going on/off/on/off just to heat up a little bit of water. It also may or may not turn on because the thermostat is not as sensitive as the smaller heaters, causing fluctuations again.

I'm not really sure what to say...it's either risking your fish or breaking your heater :(

EDIT: Okay I just saw your post lol... I would say it shouldn't take too long to heat up the 1 Gal...I'd give it maximum 2 hours on the safe side. Just check it constantly to see how fast it takes to heat it up a few degrees. If it takes literally a few seconds to bump it up something like 4 degrees, then it is probably overheating >_<