How do instincts come into play in poker? and how often should i rely on it?

What you call instinct in poker is actually your lower brain, which is emotional and reacts very quickly to situations learned from past experience (what we call "feeling" or "gut reaction" comes from your lower brain). Your higher-level brain is responsible for what we call "thinking" - ie, reasoning things out in a logical fashion, processing symbols and language, etc.

Your lower brain is trained by operant conditioning - ie, by actually doing something repeatedly until it becomes a conditioned response or habit.

In poker, your "instinct" for the cards is as good as your experience - if you have done a lot of the right actions in the right situations, you have trained your lower brain to recognise those situations and respond correctly. But if you have made the same mistakes over and over, then you have trained your lower brain to make those mistakes.

That said, we already have a lot of "instinct" for live play due to life experience - our emotional reactions to other people face to face have already been conditioned - so we "instinctively" "feel" when people are lying to us, for example.

So how much you can trust your instinct depends on how well you have trained your instinct.