Memorizing Surah Ar-Rahman, which has 78 Ayahs, can take around 15-16 days if you memorize 5 Ayahs daily. This pace is manageable for most people, but the exact time depends on your ability and familiarity with the Surah. If you’re new to memorizing or not familiar with Surah Ar-Rahman, it might take longer, possibly more than 20 days. 

The key is to stay consistent and focus on quality, not just speed. Everyone learns at their own pace, so don’t rush. With regular practice and repetition, you’ll memorize it smoothly, even if it takes a bit more time.