So I went through the books and put a timeline together. It look like about 7 years have passed between One for the Money and Tricky Twenty Two. Based on info in the books it looks like this:
One for the Money: happens in August
Two for the dough: late October
Three to get deadly: January
Four to score: July
High Five: October
Hot six: March
Seven up: April
Hard Eight: spring (same year?!)
To the nines: Early June
Ten Big Ones: August
Eleven on top: End of September
Twelve Sharp: Summer
Lean Mean Thirteen: End of February
Fearless Fourteen: May
Finger Lickin Fifteen: September
Sizzling Sixteen: June
Smokin Seventeen: Early September
Explosive Eighteen: couldn't find a reference
Notorious Nineteen: Middle of the summer
Takedown Twenty: Middle of the summer (same year?!)
Top Secret Twenty One: August
Tricky Twenty Two: Early September
The info is based on the time mentioned in the books or context clues (e.g. I want to be a June bride! That's next month, etc). Hope this is useful :)