How much time should the oven be preheated to reach the desired temperature? To bake cupcake recipes that say 180 C, how long should the oven be preheated and which level must be set among LOW, MED-LOW, MED, MED-HIGH, and HIGH?
This is difficult without knowing what sort of oven you have but here are some basic notes that will help you get started.
Many modern ovens will alert you to when they are at temperature. Every oven is different. My oven takes about 5 minutes to preheat but my dad's takes about 10.
If your oven doesn't have an alert, - and, honestly, even if it does - you should get an oven thermometer so that you can see when the oven is at temperature as many ovens do not run at the temperatures they claim to.
If your oven doesn't have specific temperature settings, only levels, it's essential to get an oven thermometer. If I had to guess, I'd say that you'd probably want medium... or possibly medium-low but only a thermometer (or possibly your oven's user pamphlet) will be able to tell you with any reliability. Also, if you only have five options, you may not be able to get it to exactly 180 C but if you're within 5 degrees on either side, you'll be fine, just watch the clock.
Something like this:
I have been cooking/baking for 40 plus years, If I bake a sponge cake or cup cakes I sometimes preheat my oven other times I forget. It doesn't seem to make much difference, they always turn out great. Just don't open your oven for 10 minutes if baking cupcakes or they'll 'fall' if baking a large cake don't open the oven for 15 or even 20 minutes.