Industry standards for Help Desk support

There is not an industry standard that I am aware of, but the situation you describe is about as bad as I've ever heard of. If the company has you spread this thin, (I personally do the same functions that you describe for a total of 20 users and beleve me this is a full time job as it is)and they will not give you any indication of when they will be able to hire you more help, they are probably not going to. THis is an unfortunate reallity in this field. Many peoplr in top managment do not know what is required to support a network, and are not intrested in learning. They have a tendency to think of the IT section as a one time expence instead of as a reacurring fact of doing buisness. My best advice to you is to make sure you have copiesof your good reviews, and hit the streets. There are alot of other jobs out there, and quite a few of them will treat you as a human being. One other point is to NOT, repeat NOT, use another job offer to try to get more money / better conditions outof your current employer. Even if they do cave I guarentee you will be just as unhappy again with in 6 months because they WILL expect more work out of you for the increase, and the changes they make, if any, will most likley not be eanough to ease your situation to a managable point any time in the near future.
Don't give up on the field, just the bad employer.

I know there is some magic number out there you are talking about.. Like 150 users per backoffice/frontoffice support person. I'm on a contract now we have a few folks spread 200-300 per person (Big staff thou).

I'd put into motion a request for a more defined SLA (Service Level Agreement). IE: These items are 4 hour response (Down server), these items are 8 hour response and these items are this week. After that I'd get a list of what exactly is expected to be covered. Start pulling some documented cases of security problems w/o support staff to handle them. Lack of proactive stuff will kill you in the long run and overburden your plate. You can look at passing some work load backup the chain. Like Account stuff (Depending on your setup).

Look into another position elsewhere if your employer is that out of whack with user needs. I hope you get paid nicely or at least have some decent benefits. Pull out some salary reviews in trade magazines and show your boss you havebeen doing your homework. :) Local Newspaper Ad's help too. Worked for me one year when I picked up a couple certs and a bunch more work.

good Luck!