How much of the 401k should you rollover to IRA?

Further clarification. Deductibility of IRA contributions when eligible for a retirement plan (pension, 401k, etc), is based on income, just as if you don't have a retirement plan. The income limits are just far less if you are covered under a retirement plan at work.
In answer to your question, all contributions to the 401k are 401k contributions, and will be represented properly on your W-2 come tax time. Choosing to roll them to an IRA, or not, has no bearing on your 401k limits or your IRA yearly limits - you are just moving the money around. So in your scenario of contribution 16.5k to a 401k, then rolling *any* amount to an IRA, would change neither your 401k limit nor your IRA limit. You couldn't contribute any *more* to your 401k, and you still could contribute $5k, subject to income limits, to your IRA regardless of whether you roll any amount from a 401k to an IRA.

IE: Made up example. You leave XYZ corp, with a $1,000,000 401k. You roll the whole $1 millions amount to an IRA in Tax year 2012. You may still contribute up to $5,000 to your IRA, subject to income limitations. (http://ptmoney.com/traditional-ro...me-limits/)

Edit to add: AFAIK, you can only roll your 401k into an IRA when you leave a company, so you couldn't contribute to that same 401k anyway, and you should basically always roll the entire 401k into an IRA anyway - you can almost always get lower fee funds or buy stocks & bonds directly, going through your own brokerage like Fidelity, TDAmeritrade, etc, since 401k's management fees almost always come out of your return in one way or another.