Hi, Stephani.
What exactly do you mean by Environemntal SOP? are you reffering to the environmental monitoring for pathogen or any other? The general consideration is to understand how the product is manufactured (including cleaning), can the physico chemical properties of your product support microbial growth and if yes what can possibly grow in the product and manufacturing environment. The sampling will then be based on the assessment of how can it possibly contaminate your product from the environment having the closest surfaces to the product stream being sampled more frequent. Try to understand that this is different from cleaning validation nd/or verification as the intent will be to understand or provide picture if other GMP is effectively in place towards microbiological cross contamination. The norms or limits will be depending on the product category.
Lastly, perform trend analysis and communicate to stakeholders (production and engineering) the trends to serve as an early warning.