I paid a couple of decorators to paint our hall last year. They did it in two days, however it cost £700 and I don’t think they used the best materials and took the best care (there are some IMO sloppy bits of work).
In March I painted our open plan kitchen/dining area on my own. Did the prep (cleaning, filling, sanding, caulking) in evenings beforehand, and then took 5 straight days to do it. Took around 40-50 hours in total, but I did it (IMO) perfectly, so whenever I go into the room there is nothing that annoys me (things that only I would notice, but you know what I mean). Even though it was a fairly tedious exercise I’d do exactly the same again.
I used Dulux Endurance paint for the walls, and while expensive, is well worth the money IMO (if you have kids). Generally don’t skimp on tools / materials – your time is worth more!
I also recommend putting brushes and rollers of emulsion in a bag and then in the fridge. If you are glossing, leave the brushes in a jar of water overnight, dry off and then continue the next day. Decent masking tape (scotchblue / frogtape) is worth the investment.
In general, take your time – don’t rush it as you’ll balls it up and it will annoy you forevermore!