I would say that the most common service level objective is to answer 80% of calls within 20 seconds (80/20). However, there is no right or wrong answer to this question – it is dependent on how important it is to your business to answer the call. In a competitive industry where sales/customer loyalty is a major driver the SLA would need to be challenging to ensure that the vast majority of customers are being addressed quickly.
You have to bear in mind that the more challenging the SLA, the more resource you will need to maintain it and therefore there is a higher cost to serve. For example, you would need more agents to meet a 90/10 SLA than an 80/120.
So, the solution is to find an ideal balance that suits your business needs & objectives.