It is my understanding that 15 inches from the center of the toilet to a vanity/tub/shower/wall is the code minimum. You may want more...it is a personal decision beyond the 15 inches.

One exception in many local codes, is to allow a smaller distance if it's a tub next to toilet since the side of a tub doesn't interfere with elbow room (only knee room) so your body isn't as enclosed. If the side of the shower in question here is a full wall, then this is useless information, but if the side is more like a tub height, this may be useful, good to know. Defining it a tub versus defining it a shower then becomes an interesting exercise in mental gymnastics with the code people if they get involved, or asking for an equivalence. Caveats apply, take no action based on this alone...

HTH

-david

In Wisconsin it's 15" cc, for ADA it's 18" cc, 24" in front.