This can depend on the extent of the work but here is a rough guide:

If you are simply having a like-for-like replacement boiler installed, then your engineer will usually have all the work wrapped up within one working day.
Should the work be a little more complex, for example if the old pipework is not fully compatible with your new boiler, you should expect the job to take one and a half to two days.
Scenarios that involve moving a boiler much further away from the current position, fitting a different type of boiler or power flushing your heating system will require more time and you will find the process could take up to three days.

It is also advisable to make a little time with the engineer when the installation work is done and dusted so they can take you through the new boiler controls.
If you speak with your qualified Gas Safe registered engineer there will be no nasty surprises as they will provide a time scale before starting the work, so you will know exactly what to expect.
Where should my combi boiler be installed?