A same-position swap: one day
Water and gas go off in the morning. The old boiler comes out, the new one goes on, pipework is adapted where needed, the system is flushed, cleaned and dosed, and everything is back on by the evening. You lose heating and hot water for the working day and no longer.
A conversion or a move: two days
Removing a cylinder and loft tanks, re-running pipework and rebalancing the system is a two-day job — as is moving the boiler to another room, because gas, water, condensate and flue all have to follow it. Day one is the heavy work; day two is completion, commissioning and handover.
What can stretch the schedule
The same short list that can stretch a price: an undersized gas supply pipe, a flue that must be re-routed, or a system so sludged it needs a power flush before the warranty is valid. The survey exists to catch these before install day, so the schedule you are given is the schedule you get.
Handover on the last day
Before we leave: controls set, a walkthrough of the thermostat, the warranty registered in your name, and the paperwork left with you. The Building Regulations compliance certificate follows in the post.