Hi Matt and welcome to you.
You have made a pretty good start with the metformin, getting down to 16+ from 24+. Hopefully the metformin will continue to help the BG levels get lower, but you can help this process along yourself. The carbohydrate in the food we eat is what creates blood sugar in the first place, so if you want to have lower blood sugars then the answer is just eat less carbohydrate. Yes, it really is as simple as that.
Have a look at the starchy carbs that you eat now, things like breakfast cereal, bread, rice, pasta and potatoes, and see what you can cut down on. I don't mean not eat at all, but just reduce. For example, if you normally eat breakfast cereal then every other day have something different (like egg and bacon, boiled eggs, scrambled eggs, omelette, ham and cheese). Try one of the lower carb breads (like Nimble at around 8g carb per slice) instead of white or wholemeal at 15g+ per slice. If you would normally have potatoes with a meal. try having half the spuds but more of one of the other vegs. If you like shepherds pie, try it with mashed cauli on the top instead of mashed potato.
How long you have to get things under control depends entirely on your GP's attitude towards diabetes treatment. Most will want to see how you are doing after 3 months, and if the treatment (and your diet) is working they will give you another 3 months, then maybe another 6, etc. Many people manage to stay on a combination of metformin and diet control for years. Metformin is by no means the only oral treatment available and if your GP feels that the metformin needs to be supplemented with something else then he might prescribe a sulphonylurea or a glitazone treatment, alongside the metformin. There are also a number of newer meds that tackle Type-2 diabetes in a completely different way, like Januvia and Byetta.
Most GPs seem to take the line that, if they can't get your bloods stable on tablets, then they eventually have to resort to insulin injections. But, as I said above, you can't just rely on the meds alone - you have to be prepared to help them.
Remember that we are here to answer any questions you have and to support you as much as we can, so just fire away if there's anything you are not sure of.