Well, if my experience is anything to go by, weeks and weeks! My friend Liana and i both got Shellac manicures on a girly lunchbreak (Liana is about a million months pregnant, so needs all the girly pampering she can squeeze in!) Our manicures were done by the amazing Jenny at Fordham Soho. Whenever i've had manicures in the past they've felt rushed, the manicurists have been moody, and i've never been that impressed with the results. However Jenny took her time with us, even unboxing about 30 new colours when we couldn't choose! She was really friendly and did an amazing job, we recommend her!
As you can see i went for a mink grey colour. In case you don't know about shellac it is painted onto your own nails like a regular nail varnish. It's then cured under a UV lamp for a minute or so, and is instantly dry and rock hard! No fear of smudging as you get your purse out to pay here. It doesn't damage your nails, and really does last. I kept mine on for about two and a half weeks (i took these photos after about a week, so you can just see the growth at the bottom of my nails) but Liana had hers on for over a month!
All salons that do shellac or gel manicures will also offer appointments to remove it, which are costly. Here's a tip for you, if you use a cheap nail varnish remover that contains acetone and soak your nails in it for 5-10 minutes the polish will easily peel away - but this is not so good for your nails, so do get it done by a professional if you can.
I would really recommend it as an alternative to a regular manicure, it is a little more expensive, but it lasts for much much longer and looks really neat and shiny the whole time. I think it would be especially good for a fuss free holiday or special occasion!