30 minutes in traffic is probably about as far as you should go. However...I sometimes went farther...not wise, but sometimes I used to say 10 minutes from the next nearest customer. I could apply aps to all the lawns on the west side on the first week of the month, and all the aps for customers on the east side on the second week of the month. Which works fine if you don't have to make a special trip for a weed redo. And if you have to make a measurement on the east side when you are working on the west side..costly in driving time. And likewise if you sell the account, you may have to drive out to house again for the first app.
This means...get it right the first time...so you don't have to go back. And try to do the estimate when the customer is home so you can do a "Sell and Do". You can give the customer a one time $10 dollars off, if you can do the ap immediately and save the trip. You save--he saves.
But its always better to have your customers clustered together. Concentrate your flyers and sales efforts within 15 minutes of your base.