As a basic user without a Premium membership you can send one Icebreaker per day. After spending it, it will recharge within 24 hours and will be available again the next day.
As a Premium member you have a total of three Icebreakers available per day. If you have sent more than one, they are not recharging at the same time, but one at a time. Each Icebreaker takes eight hours to recharge, until it is ready for use again. If you have used up all three Icebreakers within a short time period, it'll take 8 hours for you to send another Icebreaker.
As a Premium member you can buy additional Icebreakers. They can be accessed as soon as your regular Icebreakers have been used up. So you do not have to panic if your absolute dream partner comes along, but your Icebreakers are still loading. As a Premium user, you can easily find the profiles you liked in the past and contact them later on.