It depends on how much data you exchange. Every symbol chart will download the history of bars you do not have. The more symbols you watch the more data you exchange.
Even when you close MT, once you reconnect it will download all information you missed to build the chart. It will require less bandwidth than the required when running but it's not that matter.
Better idea is to watch your usage for one week or a month and get your plan.
for 30 instruments and for 24 hrs.
what i wanted to know is an approximate.
Please read here : MT5 Data