It's discussed in the Green Book on page 214. For unsignalized, the rule of thumb is two minutes worth of left-turn traffic. For signalized, it is 1.5 to 2 times the number of vehicles that would store per cycle.

Many intersection LOS software packages will predict queue length.

     "...students of traffic are beginning to realize the false economy of mechanically controlled traffic, and hand work by trained officers will again prevail." - Wm. Phelps Eno, ca. 1928

"I'm searching for the questions, so my answers will make sense." - Stephen Brust

As I understand it, most signal analysis methodologies deal with an average arrival rate. The problem with storage lanes is the probability of traffic arriving at a rate twice the average is fairly high for a small number of cycles. Unless you want to allow spillback into adjacent thru lanes during some cycles, your storage capacity should be designed to accommodate the higher (2x the average) arrival rate.

The rule of thumb I use is 1 foot per left-turning vehicle in the peak hour.  For example, 200 left-turning cars per hour would require about 200 feet of storage.

AASHTO Green Book (1984) version haa a Table ix-15. Warrants for left turn lanes on two-lane highways.

[Reference]

Harmelink, M.D. "Volume Warrants for lrft-Turn Storage Lanes at Unsignalized Grade Intersections." Highway Research Record No. 211. 1967, pp. 1-18.

Interesting its not in the newer 2004 Green Book.
Did a more recent research change these warrants?

Steven A Guinter, EIT
Highway Designer & GIS Speicalist