It's hard to say as it can differ from tank to tank. If you post some water parameters like pH, Ammonia, Nitrites and Nitrates then i'm sure you'll get some informed opinions from other members and hopefully you will get an idea of how long it will take
If you are experiencing nitrite spikes, what I would recommend is nitra-zorb. This is made by API and costs around 14GBP to treat 50 galls.
It is a sachet/pillow containing a resin compount that literally absorbs nitrite/ammonia/nitrate from your water. It can be recharged.
(Please note I have no affilation with API or any of its subsidery companies)
I would not reccommend this, if you are doing fishless cycle .
The time it takes to cycle a tank is not for no reason.
Be patient the tank will cycle on its own, using chemicals to remove things during the cycle will hurt you in the long run. When you need that beneficial bacteria, and it is not their, you FISH WILL SUFFER! Can anyone back me up on this?