Most of the jobs I've seen with Senior in them seem to want 5-7 years experience.
However to my mind that is not very senior, so I reckon that should be a minimum.
In my current team (all seniors) the average amongst 6 guys is probably 12-15 years.
We are all RHCEs, but I that is more because the organisation sends us on the training.
In most cases I think we would have all passed the RHCE without much training - i.e. we were all at or better than that level already.
What I'd expect from a senior engineer is:
1) technical ability - doesn't have to know everything but has to know how to approach problems and how to find out what they need. i.e. must have good problem solving/fault finding skills and know how to find and assess specific knowledge quickly.
2) a very good, strong grasp of the basic tools - whatever they are for your platform. They know what the standard tools are and know what they can do. May not know every option of every command, but they know most and are not surprised by anything in the OS portion of the filesystem.
3) a level of maturity that allows them to collect and weight evidence and then accept whatever will allow the fastest, bestest solution. I.e. will look at problems and solve them without ego.
4) will take ownership of problems
5) the ability to manage themselves and perhaps one or two other people - not full blown team lader or project manager type management, but if given a task that involves say themselves, a junior and someone from another team, the ability to do what is necessary from a non tech view to get the job done.
6) willing and able to share their knowledge and skills with other people within the organisation, whether that be other system engineers, app people, helpdesk dudes, whoever.
7) ability to prioritise
8) sufficient experience to be able to critically assess a system, either existing or proposed. e.g. be able to look at a system and say, "well to get the IOPs needed for X many transactions were going to need N spindles, and we'll need to have at least Y paths active to sustain transfer that over the iscsi network"
9) a very high level of knowledge of the ancillary technologies that make your systems work, e.g. LDAP, AD, DNS, TCP/IP etc.