What does felony charges pending?

Formal charges have not been filed. The formal charges are pending.

"pending (comparative more pending, superlative most pending)

1.awaiting a conclusion or a confirmation
2.begun but not completed
3.about to happen; imminent or impending
"
http://en.wiktionary.org/wiki/pending

Howlong can they keep it 'pending' ?

Depends on the statute of limitations for the particular felony involved.

And there may be other rules involving grand juries, etc.

What is the jurisdiction?

Wayne Michigan wayne county

The issue appears to depend on what the suspected crime is.

767.24 Indictments; finding and filing; limitations.

Federal and State constitutions also include the right to a "speedy trial". In Michigan, there doesn't appear to be a hard and fast rule on this, but, where a suspect hasn't been arrested, a motion to dismiss on this ground can be made after six months. http://www.sado.org/motions/chapt13.htm