Robert's Rules require that you be recognized by the presiding officer before taking part in a debate of a motion before the group, but obtaining recognition shouldn't be too hard (and if it is, you may need to raise a point of order.)
Unless your group has adopted special rules of order, Robert's Rules says you come to every meeting entitled to speak twice on every motion, with a limit of ten minutes per speech. That's 20 minutes per person, per motion, per meeting. It's a wonder meetings don't last for weeks!
In reality, your group always has access to one of the most useful subsidiary motions: the motion to limit debate. It's not uncommon for a group to adopt a motion to limit discussion to a set period of time, to a set number of speeches for or against, to a shorter time per speech, or to some combination of these options.
In order for you to be sure you have your say, you need to be familiar with how the limitations on debate work.
Speaking a second time: Under Robert's Rules, you can't speak a second time until everybody else who wishes to speak has done so. This means that if Mr. Smartypants has spoken once and the chair lets him speak again while you're attempting to be recognized, you have every right to claim the floor, even if you have to rise to a point of order to do so.
Requesting an extension of time: The chair is responsible for letting you (or any other member) know when your time is up, and it's your duty to honor the chair's polite notice that your time has expired and immediately conclude your remarks. If you need more time, you can ask for it, or the chair can, if he deems it appropriate, offer the members the opportunity to consent to an extension.
Yielding time: You can't transfer time. When you yield the floor, you waive your remaining time, but that remaining time doesn't get added to another member's time. Yielding for a question counts against your time.
As with many elements of Robert's Rules, the limits on debate aren't completely without exceptions:
Committee reports: Time limits don't apply when a committee report is being given. But if the reporting member speaks to a motion to enact one of the committee's recommendations, that discussion is on the clock.
Making secondary motions: Introducing a secondary motion isn't considered debate on the pending motion unless a member goes into the merits of the pending motion while making the secondary motion.
Debating secondary motions: When secondary motions are debatable, limits on debate apply anew to debate on the merits of the secondary motions, but debate on the secondary motion may not go into the merits of the main motion except for the motion to Postpone Indefinitely.
Sessions of more than one day: If a member exhausts his rights to speak on a motion and the motion is carried forward to a meeting on the next day, his rights to debate the motion are completely restored.