I tried looking for a solution in their documentation, but I couldn't find anything. Perhaps I might've been searching the wrong way.
Anyway, if I would add $100 to my SMS balance, and I never send a text message (or at least less than $100), will that (remaining) balance ever expire?
Specific example: if I want to send 1 message per week, can I just put €50 on my SMS balance and use it for eternity?
For SMS, pricing is usage-based, or pay-as-you-go. However, you should think about monthly recurring costs (MRC) such as phone numbers. For instance, when you buy a phone number, keeping it will result in monthly payments.
For more info, check out How Twilio Billing Works.