You'll soon find that most "turtle specific" products are not really up to par for what a turtle tank needs. The only real exception to this are the "turtle docks" but they really only work for smaller turtles.
Not only are the waterfall filters space consuming, but they just won't compete with the amount of waste your turtle will be producing. Do yourself and your turtle a favor and exchange it for a better equipped option.
You'd be wise to take this time to just completely upgrade his set up instead of doing costly upgrades in between. Trust me, all the additional tanks and filters and heaters add up

. Buy the largest tank you can now, and outfit it with durable and quality filters and heaters, it's really the best investment you can make towards a good turtle habitat.
And as for how long you can expect that 10g to last- my turtle was in it about 2-3 months max I think, and towards the end she was really too large for it. Ideally you want about 10g/inch of turtle.
Read through the habitat sections and general care on the main site, they are packed with good turtle info and ideas on tanks.