If the filter had some alive bacteria that survived the change of ownership of the tank to you, they will multiply rapidly once they are in warm water with a plentiful source of food (the ammonia which feeds one type directly, the resulting nitrites feeding the other type). It is quite feasible that you could see your first "double zero" reading within a couple of weeks.
Once you get that first "double zero" (no ammonia or nitrite), continue to add ammonia upto 5ppm for a "qualifying week" and if all is still good, your tank will be ready for its first scaley friends!
Have you and your family thought about what sort of fish you want in your 180l? As a very general rule, your "must have" fish can be anything upto ~8cm excluding the tail (known as "SL"). Once you know your "must have" and know what temperature; amount of current it likes; its aggression level; if it usually stays at a certain level in the tank etc. , you can then research other fish that will suit that environment that are available in your local fish stores.
Its well worth visiting as many Local Fish Shops (LFS) as you can, to get a feel for which hold the healthiest looking fish and helpful staff (but always try and find further sources of information for anything they say, after all, they are there to try and sell you fish and equipment). This might also open your eyes to fish you did not even exist, it certainly did for me and is why I chose riverine fish of an oddball or not usually stocked nature.
Once you think you have decided on the "must have" and suitable tankmates, you can then plan which order to add to the tank every two weeks (as long as water tests and current fish are good, ideally using a seperate quarantine tank for at least two weeks), taking account of the sensitivity and aggression of the fish.
But first and foremost, keep the "fishless cycle" ticking over nicely and I suspect you may have your qualifying week done by October
