Rule of thumb is about 17" from the motor centerline, but on some boats that is not possible. If that is the case, mount it as far away as you can. If too close, the motor could develop hot spots or overheating problems due to aerated water being picked up by the water pump. A disturbed water flow can also affect ventilation in the turns, limit the trimming up angle, and possibly cause cavitation burns on the prop.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.