Point of taxation - wcs of contractor (repair & restoration of high rise bldg)

we are in business line of repair & restoration services like building plaster, plumbing, painting, waterproofing etc. what is point of taxation ie when to discharge gst? we carried out work, after cheking & certification of work & bill from consultant, client released payment. If consultant do not certified our bill for longer period say for 3 yrs, when to discharge gst?