Most salon diaries run on durations that were guessed in an afternoon, years ago, when the software was first set up, and every appointment since has inherited those guesses. A cut and colour booked for 90 minutes that actually takes 135 doesn't stay a 45-minute problem: the next client waits, the stylist skips lunch, and by two o'clock Saturday is being run on apologies. No amount of skill at the chair fixes a wrong number in the diary.
Where the wrong duration ends up
An optimistic duration takes its 45 minutes from whoever comes next, all day, every time that service is booked. A padded one wastes the gap instead, holding twenty bookable minutes empty on every appointment, which across a six-chair Saturday quietly becomes a client or two the diary refused for no reason. Both errors are invisible in the moment and expensive across a year, and both trace back to a number nobody has looked at since setup.
Booked time and hands-on time are different
A colour appointment contains stretches where the stylist isn't touching that head at all: development, toning, processing. Set the service up as one solid block and those 35 minutes read as busy, so the diary hides capacity the salon actually has. Set the processing gap up properly and the stylist is bookable during it, which on a colour-heavy day is worth an extra client or two without anyone working faster. This is the single most valuable timing fix in most diaries, and it's also the natural home for a quick add-on at the basin while the colour develops.
Building a service properly
A service is really three or four numbers: hands-on time, any processing gap, finishing time, and a changeover that reflects reality, since nobody walks one client out, sweeps, and gowns the next in zero minutes. Variants matter too. "Balayage" as a single two-hour service fits nobody, because thick waist-length hair and a collarbone bob are different jobs, and a long-hair variant with its own duration and price stops the diary pretending otherwise.
Where the real numbers come from
Your own history, and nowhere else. Compare what each service was booked for against how long it actually ran, per stylist, over the last couple of months, and the diary itself will show you the pattern: which services always overrun, which always finish early, and which stylist the estimate fits. A senior colourist who does a balayage in two hours and a stylist two years qualified who needs two and three quarters shouldn't share one duration, and most systems can set timings per staff member for exactly that reason. Fix the always-over services first, because those are the ones taxing every Saturday.
What online booking does with your timings
Online booking turns your durations directly into the availability a client sees, with no receptionist in between applying judgement. Too generous, and the system offers fewer slots than the salon can serve, so you sell empty air. Too tight, and a client books a 3pm balayage the diary can technically hold and the salon can't deliver, and she finds out at 5:40. If bookings arrive at 11pm, and with deposits attached they should, the durations are working unsupervised, which is the strongest reason to make them true.
Gating what can be booked online
Some things shouldn't be bookable by anyone with a phone: colour that needs a patch test 48 hours ahead, corrections that need a consultation before anyone knows the price, services only two of your stylists do. Set those rules in the software, as requirements rather than hopes, because a note on the booking page saying "please call first" is read by roughly nobody. The rules are also what protect a fully booked Saturday from a stranger booking a job the day can't absorb.
Buffers, breaks and the end of the day
Three timing decisions live outside the services themselves. The last slot of the day needs to fit a service that can't overrun, because a 5:30 balayage in a salon that closes at 6 is a promise to somebody's evening that the diary has no business making. Lunch needs to exist as a blocked entry, since a break that's merely understood is the first thing an online booking lands on. And the final fifteen minutes for the till, the towels and the sweep-up belong in the diary too, or closing time drifts later every quarter without anyone deciding it should.
A review worth doing twice a year
Print the service list, sit down with the stylists for half an hour, and go through it: which durations are wrong, which services need a long-hair variant, which timings differ by stylist, and which rules online booking is missing. People speed up, techniques change, new services bed in, so the numbers drift, and twice a year is enough to catch it.
None of this is glamorous, and all of it compounds, because every appointment for the next six months inherits whatever numbers that half hour produces. The diary runs the day either way; the only question is whether it's running on this year's numbers or on an old afternoon's guesses.