Responding to "sorry I've been busy" depends on the context of the conversation and your relationship with the person. Here are a few ways you can respond:
Understanding and supportive: "No worries at all! I totally understand how life can get busy. Whenever you have time, just let me know. Take care!"
Casual and light-hearted: "Haha, I know the feeling! Life can be hectic sometimes. Whenever you're free, we can catch up."
Expressing concern: "It's alright, but I hope everything's okay with you. If there's anything I can do to help, just let me know."
Suggesting another time: "No problem! Whenever you have a moment, I'd love to chat/meet up. Just reach out whenever you're available."
Sharing your own experience: "I get it! I've been pretty busy too. Let's find a time that works for both of us to catch up."
Reiterating your interest: "That's fine! I'm still interested in talking/meeting whenever you have the time."
Remember, life can be hectic for everyone, and sometimes people genuinely get caught up in their responsibilities and commitments. It's essential to be understanding and not take it personally when someone says they've been busy. Offering support and flexibility can go a long way in maintaining a positive and understanding relationship.