There's no 'official' figure for annual progress through sub-levels of attainment.
An 'on target' child will probably be around level 2a at the end of year 2 and then around level 4a at the end of year 6. Since that transition requires moving through 6 sub-levels over 4 years, that averages out at 1.5 sub-levels per year.
However, for example, an infant school might (possibly wisely) choose to concentrate on 'breadth', rather than 'depth', within the curriculum, so that an able child is only at level 2a (rather than a possible 3a) at the end of Year 2. When that child transfers to a junior school which places more emphasis on the depth of the curriculum, teachers will be hoping that he/she will reach level 5a by the end of Year 6. So that's 9 sub-levels across 4 years, averaging 2¼ sub-levels per year.
Remember, as well, that children's attainment is assessed separately for each subject area. Many children will progress faster through the sub-levels of one subject than another.
See here:
http://www.bbc.co.uk/...y_stages_levels.shtml
Chris