When can I apply for citizenship?

You can apply for citizenship once you have been a permanent resident for 5 years, of which time you have to have been physically present for 30 months.

While you would otherwise have been eligible to file for citizenship after you have been a green card holder for 5 years since 2008, there are some concerns that you need to address before you actually file your N-400 application for naturalization. Amongst other requirements, you must also demonstrate that you have been physically present in United States for at least 36 months out of the last 5 years immediately preceding your N-400 application. Second, if you there was anytime in the last five years that you were in the US for only 2 weeks, there may be concerns whether your residency has not in fact been broken. ( a sharp eyed Border Inspector would have noted this during one of your trips into the US) Finally, please review your assumptions about whether as a legal resident your tax reporting obligation to the US is restricted only to income you made inside this country. I happen to disagree with you on this point. Before you decide to file for citizenship, please, speak to an immigration lawyer.

Since you have been a permanent resident for at least 5 years, you can apply for naturalization if you have physically resided in the United States for at least 2.5 years during the last 5 years. The non-filing of tax returns should not affect your application unless you were legally required to pay taxes and file tax returns.

You will need to meet the minimum physical presence test before you can apply for citizenship, and it doesn't sound as though you will be anywhere close to that. More concerning is that you may not be adequately maintaining status here in the US and your status as a green card holder may be at risk.

1. January 2015. 2. Yes. You must report your worldwide income. Do not file as a non resident.