New members join all the time and many arrange an exchange within a couple of weeks. The most successful exchangers are the most proactive members who send out plenty of offers.
As a general rule, the more flexible your plans, the greater your opportunities. So, looking for a fortnight in Spain during the school holidays will be easier than looking for a specific week in a specific town.
Generally speaking, short-haul trips are planned 3-6 months ahead with long-haul trips being planned 6-9 months ahead or more. The two main periods of planning are from September when the organised members start looking for the following summer. It then goes quiet over Christmas and then there is a second peak in January when a lot of new members join and activity is at its highest.