There is 20 ticks per second.
That means that 1 tick happens every 0.05 seconds.
Above info is used a lot when making recipes with CraftTweaker. As many machines needs to know how many ticks a machine should run for with x amount of power. Like for some GregTech machines you need to use:
Which then means that the machine will run for 40 ticks which is 2 seconds, and every tick it will use 120 EU, which is a total of 4800 EU.