That's a bit of a "how long is a piece of string" question! Whereabouts will you be, when will you be there.
My own view having done loads of cycle touring in the UK (incl 2 x E2Es) and Europe is to plan and book accommodation ahead - so my days/distances are sorted - with the fall-back of a credit card and taxi for emergencies! A couple of times I've just booked a day or two ahead of where I was.
Lots of online booking options - although I usually look up and find places on, e.g., booking.com and then Google the place and book direct which tends to please the owner avoiding the penal commissions the booking sites charge. The chains of hotels have central booking (Premier Inn, Travelodge, Accor - for IBIS, Novotel, Mercure) Just beware, especially in Scotland for example - where the place says it is may actually be 20 or 30 miles from there .... OK in a car, not so good on a bike!