I have several landlords as clients who have only just discovered the joys of EICRs so I've been doing several lately, and you really cannot predict how long one will take before you start. I've had studio apartments take all day and houses take 3 hours.
Some things that definitely make an EICR take longer:
1) tenant in residence (even worse if actually present and bonus points for kids) - means you can't leave wires hanging out between testing even if you know they are dead, or turn off and yank every cable out before you start, but have to work circuit by circuit - only bonus is that you might get a cuppa when the kitchen circuit goes back on!
2) Storage heaters/Economy 7 or 10 - Adds extra circuits and complications. In theory they are just single spurs, but there might be 5 extra circuits. I've just had a 10yo flat with 3 storage heaters and top up immersion on some sort of ring circuit with a central wall mounted thermostat to control - no longer works of course and the thermostat is long obselete. Not to mention it takes time to work out which circuit does which heater, and then with some heaters which is the off peak and which is on peak - because they are never labelled!
3) Idiot landlords who get you to do the EICR at the last minute after the place has been decorated, so that every light switch and pendant is painted to the wall/ceiling
4) "ghost" circuits - that either aren't labelled or supplied an immersion that is no longer there - but are still connected, though with no obvious load on testing. You either have to FI them, or disconnect them and hope they aren't doing something crucial.
5) Flats where the supply fuse is in a cupboard 3 floors down, which needs a key, and then the fuses aren't labelled properly anyway.
6) The absolute worst are old wiring ones where things have been 'added' by ambitious DIYers (Kitchens are the best - with split rings or jbs behind plinths ). And the easiest are probably the oldest - 4 or 6 wylex rewireable fuses are so much easier than a 90s 1 RCD on everything for a quick Zs on lights.
The most sensible pricing systems I've seen for EICRs is a fixed base cost, then a fixed amount per circuit - it does at least take into account the difference between 4 and 14 circuits, though harder to explain. Fortunately most of my clients understand when I say it will vary depending on how long it takes.
I did have a landlord expecting me to do 7 flats (all in one building at least) in a day and wasn't impressed when I quoted 3-4 hours per flat and 100-140 per certificate...