The answer to your question might well depend upon how big the team of people doing the job is.
Replacing a boiler with (roughly) like-for-like can often be done in a day by one or two guys working on the job. If they've got some colleagues replacing the radiators at the same time then it's possibly just a single day's work for everything.
However if the guys (or gals) doing the boiler work are going to be the same people who replace the radiators then, obviously, it's going to take longer to complete all the work.
There will be people on AB who know vastly more about plumbing than I do but, as I see it, replacing furred-up radiators without replacing the furred-up pipes that feed them would appear to be a lousy way of doing things. (i.e. I suspect that the pipe work might well be replaced too).