So consider what you can do within 5 to 10 minutes.  These should be the majority of your offerings.  For the sake of the art however, offer a couple of options that you can do in 15-30 minutes.

In order to keep the timing as short as reasonably possible, you should charge more for people with very long hair than for people with shorter hair, depending on the style.  Longer hair takes more time to do.  So if a person with longer hair wants the full length of their hair braided for a half or full head of cornrows or individual braids, add $5 to $10 to the price.

Also, even though they are simpler, individual braids take longer than cornrows to cover the same area on the head.  You should charge more for individual braids than for cornrows, for a half or full head.  The reason for this is that they are taking up chair time that could be spent on other patrons for something their friend could do for them or that they could do for themselves at home.  10 medium width cornrows can cover a full head.  To cover a full head with individual braids will take you at least 30 braids, and takes 45 minutes to an hour...more for longer hair.

Yes, the idea is to discourage people from asking for too many individual braids unless they are willing to pay you seriously for your time.

In a festival you should charge per braid for braids with extensions.  The majority of your patrons will likely want one or two braids with interesting colored or textured synthetic hair, because most people don't want to sit for hours to get their hair done while enjoying other attractions.

Try to have some pre-sectioned extensions ready.  You can buy prepared hair and braids, or you can make them at home using kanekalon hair.  Simply take some hair, twist it ever so slightly, and dip it in boiling water for 5 to 10 seconds, and hang it over a coat hanger to dry.  You can even curl it this way by wrapping it around a perm roller first.

Now let's get into festival worthy cornrow patterns...