I'm having a hard time coming up with a solution for this. I'm working on a travel site, and one question is "How many days do you want your trip to be?"
I want to handle two cases, "7 Days" or "7-9 Days".
Having two fields is a bit awkward if they know exactly how many days they want. But can anyone come up with a cleaner, intuitive solution?
Since there are too many options that users can enter, why not give them freeform entry field?
ITA's (now, Google's) Matrix does just that:
It even has a limit to how many nights can be selected:
However, their solution isn't perfect because users are notified of the limit only when the field is moved out of focus and some people will submit the query without changing other options. Thus, it's better to validate it in real-time.
They also handle the results of such a search very well showing on the lowest price for the departure date, graphically showing trip duration on the calendar, and giving quick access to other trip lengths starting that date.
Why not just go with how you've described it? Allow them to type either a single, or a range of numbers into the single field? Similar to how Microsoft Office print dialog works; you can specify single pages or a range of pages within the same field.
This concept has been around for a reasonable amount of time so isn't a new type of request to throw at a user. Provided the field is annotated correctly this should suit your situation.
On a related theme, Hipmunk uses a -/+ button to extend the date from -2 to +2 days from that entered - thus using a single field and an optional use of the button to broaden the range. Very nice feel to it - especially with the visual feedback on the calendar view.
