A friend asked me how a writer knows when he or she is ready to start submitting a manuscript. The following is my response:
There's no magic formula that will tell you when your manuscript is ready to send. The only thing you need to know is if you're ready to send it. That won't be until you're ready to accept rejection, which is inevitable. Your chance of success is only something like .01%. Stephen King could wallpaper his office with rejection letters before he sold his first novel, and it wasn't the first novel he wrote. Even Jane Yolen still gets rejection letters. When you know down deep inside that rejection happens to the best and isn't a reflection on you as a writer or on your manuscript, you'll be ready to start submitting.
According to a survey conducted by F/SF writer Jim C. Hines, it takes a writer on average 10-11 years of hard work to get his first novel published--and it's most often the author's 3rd or 4th manuscript. If it takes one year to write the first and two years between the time a book is purchased and published, that means 7-8 years of rejection letters.
If you're ready to start the ball rolling, then you're definitely ready to start submitting. You might luck out with your first try, but if not, you'll be on your way. I've been submitting for about 7 years now, and I think my 5th manuscript will finally get me in the door. My first is my magnum opus, but I think my 6th will be my bestselling. I have a hunch it's my most marketable work yet.
I hope this helps and isn't too depressing. Do check out that survey. There are definitely things you can do to help. Go to conferences and workshops. Join a critique group or two. Read a lot in your chosen genre and really study it, analyze what it is you like and don't like in a particular book and why. Read books on writing and editing. Write and edit, write and edit, and write and edit some more. Don't settle for writing just one book. Write another and another. Submit to agents, publishers and contests. And always remember that slow and steady wins the race.
All the best,