A 300 litre cylinder is very well insulated, Rocky. It will hold its temperature for quite a time...... but .... I wouldn't restrict it to just one top-up per day. You're going to use a lot of energy, plus quite a long time to heat such a large hot water store if it cools down.
One longish period in the morning, and again in the evening at least. For households that use a large amount of hot water, it's well worth leaving it on all the time. As I mentioned above, it takes some time to heat up a large cylinder from scratch.