I read the lyric section,but rarely post,so my observations will be less valid than those who regularly contribute.
As Sing4me88 says,the chances are,a lyric will take less time than the rest of the song,so on the surface a week is a fair rule of thumb. This is not to denigrate in any way the fine lyricists we have on the forum. I know sometimes words can be agonised over for much longer periods.
Another point worth considering is,a lot of musicians find it easy to compose,but finding meaningful lyrics takes longer and can be more troublesome. To my mind it's a bad thing to discourage any member who may be lyrically 'on a flow' and force them to fit into a set template all of the time. What I'm saying is,rather than a strict rule,a quiet word with someone may work better for the forum.