How much should I invest in stocks?

A lot of factors go into this answer. I think a reasonable goal for retirement savings is 20x your annual income by the time you retire. This would allow you to withdraw your annual salary at a 5% withdrawal rate. Over a period of time, a 4-5% withdrawal rate should allow you to meet your needs without eating into the principal balance.

You might want to shoot for a bigger number if you expect to keep increasing your income and want to prepare for a higher lifestyle in retirement. Thomas Stanley, who wrote the Millionaire Next Door had a good recommendation for where people should be on their saving plan by age. His formula is to multiply your age by your income and divide by 10. So, if you make $50k a year and are 35 years old, his model would suggest you should have a net worth of $175k. That's 35 * $50k / 10 = $175k.

I think it makes sense to save between 10-20% of your income, including any company match you get from your employer. That level of savings should enable you to meet your retirement goals with a reasonably constructed portfolio. Hope this helps and best of luck getting started on your savings plan.

-Marc