Well, it depends on the type and quality of the cylinder installed and the insulation around it (foam lagged/cylinder jacket etc). However, the average water cylinder will remain hot for several hours. If a shower/bath is taken, then of course the hot water reduces as it’s replaced with cold water. At least you have a temporary hot water backup for the odd shower. Unfortunately, with a combi boiler you will not have the luxury of a cylinder full of hot water as backup (it was removed when the combi was installed) That’s when you have to move quickly to avoid any inconvenience by calling serviceteam Ltd. and get your repair visit booked.