Central heating and Domestic Hot Water (DHW) are two different issues. The combi lobby makes a big thing about only heating the DHW that you use, and that you will avoid "standing losses" from a hot water cylinder (this being energy that is lost as the hot water inevitably cools in the cylinder). A typical modern cylinder which is quite well foam-lagged loses about 2.5 kWh per day if it is about 120 litres capacity. What the combi people fail to tell you is that whenever you turn on a hot tap or whatever, the boiler starts up to meet demand. The sequence is simple. The combi detects a demand, and turns on the gas supply,
a few seconds later it ignites the gas and the heating process begins. This is the same with any boiler, combi or not. The problem is that every time you wash your hands for ten seconds with combi heated-hot water the few seconds of pre-ignition gas is wasted. With a conventional system, the hot water boiler activity is just controlled by the cylinder thermostat which trips the boiler far less frequently and pre-ignition gas is wasted far less.
Re-heat times for hot water cylinders are pretty fast these days. About 20 minutes for a 115 litre cylinder and a 20 kW boiler is my best guess.
It is a lot more complicated than this. It depends on your pattern of DHW use, and your home. If you have electric showers, no bath, and a cold feed dishwasher and washing machine, you probably have no need for hot water from a boiler/combi apart from the odd handwash.
Nothing is that simple, alas.