@swissclive this is an excellent question actually... Let me try my best to answer this...
According to me it always depends upon which GPU hardware you are choosing to mine on your PC or Mac and respectively, how much MH/s it generates...
Since Equihash is ASIC proof, we can fairly say that there are very less chances for mining difficulties to go way up higher like whats happening with Bitcoin and Dash... Although even if does go higher it would be distributed, would not happen something like ASICs are coming with more and more speed...
Hope that answers your question, please let me know if i missed something...