how often are we aloud to advertise our site in the hosting offers section, or can we only do that once?

Everything you need to know can be found in the Rules (http://www.webhostingtalk.com/rules.php). :)

To quote the relevant rule:
Advertising is limited to one ad per 7 days posted in the Advertising Forums regarding your companies, sub-companies or associated companies. It will remain any companies' responsibility to schedule and maintain the offers your employees post in conforming with this important rule.

thank you :)