i think it is usually yes 3-4 weeks.

We have 400gm tins of our formula so it makes it easier for us.
I think with DD when we were using the 900gm tins i would often go over in time but i didnt notice or i weaned onto soy (obviously older - 12 months).

2s6 gold have sachets but they will prob work out the same price.

it is one of those catch 22;s