Welcome to the Community, @NickiG82
Alongside the great advice @XRaySpeX has shared above, when it comes to adding extra products there isn't really a one size fits all rule for when this will change.
All customers have an internal credit class, which is determined by both their initial credit check and how they manage their bills moving forwards.
My best recommendation would be to set up a direct debit so all bills are paid on time, and this will maximise the chance of your credit limit and the additional lines you can take increasing.
Peter
