I-140 was approved and then revoked in 2011. Can I keep my priority date and also apply for I-485? What is the deadline for filing I-485?

It depends on a couple of things. First of all the regulations changed in your favor on January 17, 2017. Therefore, if the revocation occurred before January 17, 2017 chances are you might have lost even your priority date. On January 17, 2017 the laws changed and now the rule is if the moment your I-140 is approved the priority date is yours to keep. The second rule is if the I-140 stays approved for 180 days you also have the right to extend your H-1B and your spouse their H-4. There are two different rights  - right to keep your priority date the moment I-140 is approved and  the right to extend H-1B if the I-140 stays approved for 180 days. Hence this is a protection even if the employer revokes your H-1B.
For more discussion on this matter you can have a look at my older blog entries.
https://www.immigration.com/blogs/

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