けっこう has many meanings but you should check it has a core meaning like 'quantity/quality is much' basically. In this case, it means 'ok', 'good', or 'satisfied'. ですので means 'so', 'since' or 'therefore'. The translation would be like this.
It's good enough for me to get the answer when you're available, so ...
「...でけっこうですので、...」 is popular phrase because it certainly asks something but will not ask forcefully. More casually you can say 「...でいいので、...」. いい can be used here as a casual form of けっこう. 「来月でいいのでレポートを提出して」 sounds like "It's ok to submit the report in the next month. So be sure to do it".
Last of all, I thought your translation might cause a little misleading. This doesn't say "it's OK if you don't have time" but "it's OK if the answer is late".