A common question amongst those about to record their first demo
is "How long will I take" alongside "How many tracks can we record".
The answer, like so many other things in life is "It depends".
Studio time has a knack of running at many times real world time.
A common mistake is trying to record before a band is really ready.
Recording is a slow and involved process, there are many factors
involved in creating a good recording, the first and foremost is
capturing a good performance. The skills called upon are not
always the same as those needed for a good live gig, though having
experience of playing live will certainly help.
Another common mistake is trying to record too many songs in too short a time. If the aim of your demo is to get noticed, then getting a few tight well arranged tracks down is a far better investment than trying to do six tracks that all sound rushed. Reviewers are busy people and some will make a point of simply chucking demos that have more than three songs on. That first song has to be excellent, otherwise the others are not going to get a look in.
Mixing is quite a separate process from the recording and needs to be factored in to the day. CDs take time to prepare and burn, although it is a lot faster than it used to be, there are some tasks that still take time and cannot be rushed.
As a very rough rule of thumb, an experienced band that are well rehearsed and know exactly what parts they are going to play can record up to three songs in a day, depending on how long the songs are, how many instruments there are and how much sleep they've had the night before. This doesn't take into account mixing, overdubs and a good critical review.
Recording is an expensive business. Make sure you and your instruments are in good shape (that includes voices!) before getting to the studio. Spending a fiver on a new set of guitar strings can save hours of expensive studio time in trying to salvage a decent sound. Don't bring in a new amp or other equipment that you're not familiar with. Stick with what you know.
Although studios can be very creative places, rehearsal time is a lot cheaper, so learn your parts first, "improvising" on studio time can be very expensive and time consuming.