How long until a tooth comes up?

my 3 month old always puts his fist in his mouth and grabbing his bib 2 put in his mouth i got told that its because his gums are at him then i had said it to health visitor and she said he is teething but i was just wondering how long will it be before his tooth will come up i mean their is no sigh of a white tooth in his mouth i don't know anything about this i am a first time mom so any answer help full