How long to preheat an OTG?
When you set the oven to the required temperature and then close the door and turn it on, pretty soon you will probably see the heating element begin to glow orange from the heat. When the oven is up to the selected temperature, the power to the element should then switch off for a while, and you should be able to see the orange glow diminish and start to fade. That is when the oven is considered "pre-heated". My "big" electric oven (regular size, really) takes about 10 minutes to pre-heat, so your little OTG should be quicker -- maybe 3 to 5 min.(?), but watch the element, or, as Jolenealaska mentions, the indicator light. If it has an indicator light, when the light goes out for a while, it's pre-heated.