That's a good practice, to give your racks back to you to hold until the work can begin. It does a few things: keeps you interested in your trophy, cancels any liability on the taxidermist's part and it also frees up space in the shop.
1.5 months is an adequate time to mount, and from your post date, it is a few weeks away now. Be sure to post up some pics, we'd like to see the finished product.