There is no such thing as "visitor visa"... I would assume you mean "visitor status".
Anna999 said:
Can anyone tell me what the protocol is if your 90 day visitor visa expires. I realize one has to leave the country but what are the rules?
You mean the grace period for applying for restoration? This is tricky. If you run out of status and do not file for an restoration, then it is assumed you will leave the country immediately (tops 1-2 weeks) if not then it will be considered overstaying.
Anna999 said:
Is driving across the border to the states enough to gain a new 90 day visitor visa,
May or may not. Up to the CBSA officer. CBSA officers are well trained to spot "smart people" who want to skip the line of proper restoration applications.
Anna999 said:
how long does one have to stay outside of the country and
No such thing.
Nonetheless it is advised at least the same amount of time you spent in Canada.
Anna999 said:
how many times can one renew the visitor visa in say an 18 month period?
You cannot renew a "visitor status". You have to extend it.
How many times and duration depends on your specifics. Technically there is no limit for number of applications nor time given.