Time to heat water on electric source

We've been preparing for our first trip in the caravan today by testing out the water and heating systems. I didn't switch on the gas, just used the mains hook up to power the boiler, which is a Truma combi.

Just a question of how long it normally takes to heat up. I left it on for 15 mins, conscious of not using too much power, in these times of astronomical energy prices. I think it was working, but I suspect it needs to be on for a lot longer to heat up fully. 

I guess using gas, it provides more or less instant hot water, although we've not tried switching on the gas yet.