It depends on the situation, the individual and the relationship. My Guardian Demon often comes on Her own and we talk a lot. The summoning ceremony is more about connecting, getting to know each other. Once a good relationship is established, the Summoning is not necessarily necessary.
Yes, your guardian demon is often close to you and can contact you or guide you to important things, often even if you cannot hear them directly, they guide you in other ways.
Mine communicates to me telepathically mostly, also by giving me gut feelings and often I see a mental image of Her sigil with these feelings and a sense of assurance. I dream with Her often and drift lucidly into these dreams quickly almost immediately after I lie down to sleep.