I'm still learning radioactivity.

 

Alfa is a particle which owns two protons and two neutrons, right?

 

So, if it is released, it may become helium, considering that the particle alfa catches two eletrons. Is what I am saying right?

 

If it is right, is there a medium time that it happens? I mean, how long does it takes to alfa become helium?

When an alpha particle is emitted from the nucleus it's going way too fast to capture electrons, but it will quickly slow down enough after many collisions with surrounding atoms. This happens very fast; when alpha particles are emitted they have velocities of thousands of kilometers per second, and usually stop after a few centimeters in air. The +2 helium ion will then eagerly capture electrons from surrounding atoms, becoming a neutral He atom. All of this happens very fast in a fraction of a second, hopefully someone else can give a proper estimate.

 

To summarize, yes, alpha particles are helium-4 nuclei and will eventually become electrically neutral helium atoms when they slow down in an environment where electrons are available. The Earth's crust contains surprising amounts of helium for this very reason as abundant radioactive metals like thorium and uranium have many alpha decay events down their decay cycle.