Reporting a felony on a rental application?

Unfortunately, there is no time limit on that. If the conviction could be vacated, then yes, you can say you have not been convicted but Assault 3rd degree is not eligible for vacation. He is going to have to keep disclosing his conviction.

The passage of time alone does not provide for your negative response on an application. You need to take appropriate action to have the felony expunged from your record.

If the question is have you ever been convicted of a felony, the answer doesn't automatically change after a passage of time--7 years is how long negative information is supposed to stay on a credit report, but background checks are much more extensive. The only way to be allowed to honestly answer no, and to have it stop showing up on background checks, is to have the felony vacated through a formal petition to the court and tell the screening companies to stop reporting it if the record is successfully vacated. However, it is unlikely that your boyfriend would qualify, because violent crimes and crimes against a person cannot be vacated: http://www.courts.wa.gov/newsinfo/index.cfm?fa=newsinfo.displayContent&theFile=content/guideToCrimHistoryRecords#P76_6824

A negative response is only truthful if the conviction has been sealed/expunged after the statutory period of time has expired.

As the other answers make clear, felonies never go away on their own no matter how much time passes. And, under Washington law, certain felonies are never allowed to be removed from a person's record. Unfortunately, Assault 3rd Degree is a type of offense that can NOT be removed ever. So, your boyfriend finds himself in the position of having to report this conviction for the rest of his life.