You need to get or do a rent survey of other properties in the area.
Appraisers typically compare the rents on a square foot basis.
Rent can be quoted monthly or by the square foot depending on the owners and the norm in your area. What is a typical term Drive by and look at the buildings and go in any rented units when comparing age and condition. Compare the traffic counts on the streets. It would be a good idea to find out from a leasing agent how long it typically takes to lease the type of space that you have. When you determined the rent and marketing time I would work with the tenant that has been there and determine what their needs and wants are.