GTM version Active Session detection by IT-Agency.ru. Create Custom HTML tag with this code firing on All pages and take event active_session in dataLayer.

active_session_detection.html

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script>
(function(e){var c=0,d={},a=0,b={init:function(f){return this.each(function(){d=jQuery.extend({achieveTime:60,loop:0,eventList:"blur focus focusin focusout load resize scroll unload click dblclickmousedown mouseup mousemove mouseover mouseout mouseenter mouseleave change select submit keydown keypress keyup error",testPeriod:10,useMultiMode:1,callBack:function(g){console.log("Achieved!")},watchEvery:1,counter:{test:0,achiev:0}},f);d.watchEvery*=1000;if(d.useMultiMode){b.loadMultiData()}if(d.counter.achiev!=-1){e(this).bind(d.eventList,b.eventTrigger);b.process()}})},process:function(){d.counter.test+=1;if(d.counter.test==d.testPeriod){if(a){a=0;d.counter.achiev+=d.testPeriod}d.counter.test=0}c=setTimeout(b.process,d.watchEvery);if(d.counter.achiev>=d.achieveTime){if(!d.loop){clearTimeout(c)}d.counter.achiev=d.loop?0:-1;d.callBack.call(this,d)}if(d.useMultiMode){document.cookie="activity="+d.counter.test+"|"+d.counter.achiev+"; path=/;"}},eventTrigger:function(){a=1},loadMultiData:function(){var h=" activity=";var g=" "+document.cookie;if(g.length>0){if(g.indexOf(h)!=-1){offset=g.indexOf(h)+h.length;var f=unescape(g.substring(offset,g.indexOf(";",offset)==-1?g.length:g.indexOf(";",offset))).split("|");d.counter.test=parseInt(f[0]);d.counter.achiev=parseInt(f[1]);return;}}d.counter.test=d.counter.achiev=0}};e.fn.activity=function(f){if(b[f]){return b[f].apply(this,Array.prototype.slice.call(arguments,1))}else{if(typeof f==="object"||!f){return b.init.apply(this,arguments)}else{e.error("Method "+f+" does not exist on jQuery.activity")}}}})(jQuery);
</script>
<script>
$('body').activity({
'achieveTime':60
,'testPeriod':10
,useMultiMode: 1
,callBack: function (e) {
dataLayer.push({'event':'active_session'});
}
});
</script>