Assuming you’re on Eight’s $8/30 days plan, a single $28 top up is like a >90 day ”commitment,” or less. You would’ve paid $32 to stay with Eight for >90 days (but less than 121 days). Instead you only paid $28. If you top up twice that‘s a cost of $56, so that’s like a >180 day “commitment.” The fact you’re receiving more than 30 days of service with this promotion per $8 of spending doesn’t mean you’re committed to using the service. You’re not really committed anyway. You’re just trying to decide whether to prepay for some $8/30 days service, and by how much. You shouldn’t count this discount against your decision.
I think about 3 top ups per line makes a lot of sense with this promo (meaning you expect to stay with Eight for at least 301 days, the “come out ahead” mark), but YMMV. For example, if your 60th birthday is getting close then you might plan to switch to SIMBA’s $5 plan then. That’ll influence how many Eight promo top ups make sense for you.