True Move Top Up promotion - When available?

[peeba7]

I get text's telling me about a 90 baht bonus (after topping up 90 baht) but only after I top up AND never before. How can I know when this promotion is available prior to topping up so I can take advantage of it when it comes around. I am referring to something like this: 'You have been with us for 12 months or more, get up to 100% bonus when you top up.' It is actually a pretty good deal if I can know before hand when exactly it is available. The bonus is good for about 5 days unless you hit in between the bonus period. Knowing beforehand would help.

[peeba7]

It happened again yesterday, topped up 90 got 90 baht bonus but only one day left, had i known would have topped up sooner for I think 5 days bonus use not just one.