Ok,serious.

If you want to play as purely F2P,you can play all the contents without spending a dime but will require alot of time and patience for you to catch up to veteran players that have been playing the game since launch (or more than 1 years).The good news is alot of contents have been revamped since past year to be beginner friendly,daily log in rewards plus a main guide questline giving new players free heroes,equipments,etc so you can start doing any PVE contents almost immediately.Not counting the events that basically give you freebies just by doing daily in-game stuffs. 

As purely F2P,I will say around 6 months to 1 year for you to catch up to veteran players (depending on how active you are).

From my personal experience,I have only spend around RM150 to speed up my progress catching game when i start playing (after 8 months,I am already self sufficient and able to tackle any current or new contents).

But if your question is more specific like "No,I am asking on the scale of 10,how P2W is this game if I want to reach absolute elite boss spanking derp top 1% players?" then my answer is "Prepare to pay until your wallet's hole is deeper than a blackhole." (Even more wallet damage if you asking about the PVP side too) 

If my reply still can't answer your question,try to play the game for 1 month,follow the beginner guides (or ask game related questions) in Seven Knights Reddit then you will surely get the answer.