Can some one please explain what is meant by timer?
Ghost sites have a timer that activates under two conditions:initiate warp to the site while uncloaked uncloaking when at the site
The exact duration varies site to site, and is not shown to you. If you take too long, then the npc's show up and kill you.
*Most (not all) ghost sites will persist for at least two minutes. I run these sites in a frigate by setting a timer on my phone for two minutes, and if I have less than about a minute and a half when I'm about to hack the next can I bug out. If I'm in my Stratios I can tank the explosion and the subsequent rats. A tanky Astero can tank the lesser covert sites, and I think there's a fit in corp fittings for it.
In a fast frigate I can reliably get 2 cans, and occasionally 3 or even 4, when I have a blackglass plugged in.
Also, be aware that someone else may have triggered the timer if they are watching you. Savvy hunters may sit cloaked on-grid (but out of range of the explosion), and briefly uncloak to start the timer when they see you pull probes, in anticipation of you warping to the site in the next couple of minutes. Or, some other explorer may have just left the site, and the explosion may be imminent. I've died this way at least twice.