Immersion heater on 300L cylinder

Hoping someone can advise. Our boiler is broken and won't be looked at until Monday.

I have a unvented 300L cylinder and turned the immersion heater (see photo) on about 4 hours ago.

The water has got barely luke warm. But, from the online pics of this immersion heater, the heating element looks very small compared to the capacity of the Cylinder.

Should I expect to wait severel more hours for hot water, or have I done something wrong.

Any advice will be much appreciated.