What Jonathan said - plus the fact that, in this case, "it's" means "it has" - "it has been a couple of months". Present perfect tense, so "have" is the auxiliary verb.

Every sentence in English needs a subject. Often when we're talking about time, distance or weather we use "it" as en empty subject-- essentially a placeholder. So, yes you need "it's (it + is)" rather than just "is." Native speakers are so accustomed to hearing sentences constructed with "it" as an empty subject that if you omit it you might not be understood.

The past participle “been” is always preceded by some form of the verb “to have”. No other past participle has this property.