One week
I sent my 70-page proposal and received a call from a savvy, veteran agent after a week. And, after a minor rewrite, he offered me representation. As you can see from my usersname, I write non-fiction sports, so the agent had to have publishing contacts that dealt with such a genre. As one agent told me, "Patience is a must in this business."
When you stop brooding over the agent skimming through the manuscript, and start mastering the craft, your chances of an irresistible proposal will increase. Many agents are a sucker for good writing.
A good place to start is reading, On Writing Well, Harper Collins, by William Zinsser. I Just finished, How To Tell a Story, Writers Digest, by Peter Rubie.
Such books open your eyes to the importance of plot, character development, scenes, and active verbs and pinpoint adjectives.