How do you know if your pen pal is losing interest in you?

He used to reply within 2 hours and now it's been 2 hours and I've gotten no response for him. About 3-4 days ago he sent me a message at the end of the e-mail saying he was running out of things to say to me.

Also I want to start a new pen pal relationship with my sponsored child in Rwanda - I can send him a 2 page letter - but I have no idea what I should be asking him or how I should talk to him.
How do you know if your pen pal is losing interest in you?
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