The washes around me do not have seasonal hours and some are 7am - 9 pm, with some 8 -8 on weekdays. Some open and close at 8 on Sunday, but that is about the only difference from the longer weekdays.
Easy for me to say you need outdoor lighting, but for the safety of employees and customers, you should have a well-lit facility. For the last hour of your day as it gets dark, you may have people not comfortable using a wash that is not well lit. In the northeast there a lot of dirty and salty cars in the winter months and there is money to be made between 5 and 8 pm in the winter months. That might be different for you though.
Do you plan to have a full time manager, or will you be an owner/operator? Do you plan to keep most of your employees as part time?
Two pay stations and prepping but no toweling can probably be handled by 1-3 employees at a time depending on volume and how extensive your prep is. It will not be static, you will need to flex with the volume. Also have to consider customers enrolling in monthly
loyalty plans and any gate issues where you need an employee to keep watch. You may be able to open with one employee and add the labor as the day goes on and volume increases. Just a ballpark shot at it given what I have to go on.