You have to remember that most of the slow release fertilisers are just that, 'Slow Release'. The granules break down slowly over a three or six month period and not straight after application.
I personally would apply now as the plants are active in growth over the Spring and would benefit from any additives/trace elements they can get.

Do prostantheras need a special native fertiliser? I have not heard that they have any particular problem with Phosphorus, so probably any general fertiliser would do.I'd imagine that something with a bit more potassium a month before flowering would help, and something with nitrogen swallowed with plenty of water after the post-flowering pruning job might be a good idea.
But go back a step further, do they really need fertilising at all? There's a big industry out there happy to convince us that algal blooms in our waterways have nothing to do with all the excess fertiliser that ordinary people like us put on our gardens. I understand that there's some pretty good evidence that timely use of just enough water produces results as good as fertilising, especially on plants like prostantheras which grow naturally on very poor soil.
Trish

Good point Trish
I grow them as they fit well on my site, being a bush garden with a lot of gum trees, (and shade) they are one of the only understorey things I can grow and in fact most are indigenous. (and I love the scent and flowers and bees)
So being pretty much as they would be in the wild except for some extra care with pruning and water - perhaps I don't have to fertise at all.

I would at least "fertilise" with compost. As suggested earlier, you could do this late winter every year before they flower. Give them a prune, then let them grow.