What is the acceptable shipping time ?

As per my comment. The best shipping time is within 2 working days, which means your customer place the order today before 12pm. He should able to receive tomorrow or the day after tomorrow. 

Within 48 hours is the best arrangement time. 

If some reason, if the purchased item is taking more than the estimated time to deliver. You must send out the official email to inform your client. Prepare to refund if they are not happy the shipping is delayed. 

Mostly the client is ok to accept the delay shipment, if you inform early. No need to worry the client cancel the order. Just refund if he/she really can't wait for it. 

The speed of the deliver is really helpful to build your good relationship with your customer. The customer will prefer to order again in your website,  if they are happy about the product and your service.