A switch rated as 30A is designed to carry 30A continuously.
But, the actual application depends on your switch and the load. Switches should always be sized based on their Make, Break, and Continuous ratings. Most times we only refer to the continuous when discussing switches, but it is not uncommon to have the Break rating of the switch significantly lower than its continuous rating (for example a toggle switch might be rated 60/6/10A) . This is why it is important to have devices rated for their load characteristics, whether it is tungsten lighting, horsepower, or resistance heating.
Most power switches (including, wall switches safety switches and contactors) are rated to break their continuous rating, however most "control" switches and relays are not.