If you were a System Admin for a small company and you're the only one, when should you put Senior System Administrator on your resume? I know larger companies who have many System Admins have different title levels, ie. System Admin IV.. But is there a general rule when you can put Senior in your title after a number of years experience? This is not limited to System Admin positions, but IT in general, like Security Analyst.
It depends on the work you do, but for most small companies where you're a one-man shop, you're probably not going to get the experience necessary to claim a title like that due to lack of exposure to many different technologies. In a small shop, you typically would perform as much (if not more) "desktop support" than real system administration. The same can be said for other lofty titles at small companies -- if you're an "IT Director" at a 50 person shop, well, that isn't really a Director-level position.
Really, you should jump to a bigger shop to get more experience and exposure to different technologies and topologies. Exposure to technologies/topologies such as domains/networks spanning multiple continents, different server hardware and software (particularly things like AD, VMWare, Exchange/Notes, BES, SQL, SMS, Shavlik products, Sharepoint, other web technologies such as IIS and/or Apache/Tomcat, various Windows and/or Linux versions), strong knowledge of client platforms, and familiarity with enterprise hardware such as SANs, firewalls, etc. Those are some of the things a truly senior person would have knowledge and exposure to. You obviously wouldn't need to be an expert on every single thing on that list, but you should be an expert in a few and at least have some familiarity with the other stuff. You should also be considered a "lead" on many projects and/or a mentor to junior people.
EDIT: I guess my description above is really a description for an Enterprise Administrator or Enterprise Architect, which is probably overkill for what you were asking.