Be sure the oil level is upto proper level...
Put even an additive to quite the lifters...??? possibly worth the small investment...!
just a guess...a loose valve retainer holder....it might just need a slight adjustment (tighter)...
It could spread to several cylinders...sounding like a sewing machine if not attended to...
Time frame would depend upon the 'usage' of the engine...light for small trips to heavy for a daily drivers for many hours. The lighter the 'usage'....miles driven....the longer time period you would have to take a peek under the valve cover(s)...both side while your at it....