Typically you can Post feed help requests to your wall once every 6 hours. This may vary from feature to feature, but the game usually informs about any change to those 6 hours.
Now for Requests, you can send as many as you want to as many friends you have once every 24 hours. The recipient then will have 24 hours to accept these Requests before they expire.
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FarmVille 2: Country Escape Game GUIDE
哈利波特：巫師聯盟