Minimum should be 2 feet from top of utility (ex: if conduit, the initial hole would have been deeper). Does not have to be below frost line or you'd be looking at like 6 feet. Overkill for something that can freeze. Water/sewer should be like 10 feet at least though as you want those below basement and below frost line. Heck we get water main breaks here all the time even at 10 feet. But that's because they use so much salt on the highway so it's constantly down to the pavement and the traffic drives the frost lower.
That said it's a good idea to get a cable locate if doing any kind of digging, it saves you from the responsibility should you hit something that was not suppose to be there.