The standards of length for vanity tops vary from builder to builder and they keep changing it over time. If you measured 53-inches that is what it is. We install granite countertops and just about every house we do is different. What you may be referring to is the standard cabinet depth, which is 22.5-inch with a 1.5-inch overhang in the front. If you are handy, you may want to go with a granite countertop installation. You can have your local company just sell you the piece already cut to fit and you do the installation.
Bathrooms are built to fit the bathtub, the toilet gets moved to fit between floor joists and then they order the cabinet to fit the hole thats been left for it, nothing standard, but the depth is usually the 22 1/2" and the top is deeper.