你真傻，我教你吧

var time=0;

var recordTime=null;

wx.startRecord({
  success: function(res) {    
    recordTime=setInterval(function(){time++;},1000);
  },
  fail: function(res) {     //录音失败
  },
  complete:function(res){
    clearInterval(recordTime);
   //time就是录音时间
}
})