Your midwife or doctor will let you know how much weight is ideal for you to put on during your pregnancy.
If you have a normal BMI they say between 11 - 16kg weight gain is fine.. I at 29w and I think I have gained around 10kg so far, but in the beginning weight gain was very slow! At this stage I think your fine :)
But remember weight gain is not all you putting on weight.. bub, fluid, placenta all add to it.