I find it is best to arrange the model casting around a week before the shoot takes place. If you can avoid casting on a Monday, then so much the better. Around the busy hair awards shooting season, there are always so many hair castings and hair shoots that take place on Mondays. If you cast on this day, you will have less chance of seeing the models who are best-suited for hair photography. Alternatively, avoid shooting at the busiest time of year (May to July) and book your session in early!
I will normally organise for a model casting to take place at a central London studio and aim to get in around 60-70 models (sometimes more) from the leading London model agencies. I will photograph the models and add their names and model agency details, inputting everything to a laptop computer at the model casting. This means that at the end of the day you can review all the models who came along quickly and easily. Where clients can't make it to the model casting or need time to make their final selections, I have used websites to make it easier for clients to sort through the models who attended the casting. There are several different types of website designs and it only takes me a few minutes to generate a site such as the one shown here.