The amount of each thread you will require to complete a project depends on several factors, primarily the number of stitches in each colour and the count of the fabric. Also, the grouping of each colour throughout the design affects the quantity required - large blocks of a single colour use less thread than the same number of stitches in small isolated groups.
As a guide, at Cross Stitch Workshop, we allow 1 metre of thread for every -
* 250 stitches worked in 14-count Aida
* 330 stitches worked in 18-count Aida
* 200 stitches worked over two threads in 25-count Evenweave
For example, suppose there are 650 stitches in one colour and you are working with 14-count Aida. Divide the 650 by 250 to get 2.6 and then round this up to 3 metres to get the quantity required.