Exam dates can fill up - it depends on many things like time of the year, wkend vs wkday and test location. But generally, you can get a sense by looking at your test center's date availability - you can see which days are booked in the weeks ahead.
At GMATPill, we recommend you to schedule your test in advance. This serves a very good motivational purpose. Once your test is set, you will force yourself to allocate the time to study. This becomes increasingly difficult after school and especially while working. Setting aside free time after work (when you would normally relax or socialize) to study instead is extremely difficult without being "forced" - at least for most people.