By default the To-Do bar in Outlook shows the current month only. If you want to see multiple months shown in the To-Do bar, you can configure this by going to View -> To-Do Bar -> Options. You can then enter the number of months that you want shown in the text input beneath the Show Date Navigator checkbox. The number of months that can actually be shown will be limited by your screen/window size, so if you pick too large of a value, you may want to lower it back down if all the months are not being shown.