According to my book, if grown from small as houseplants, these usually take about 5 years to flower, and should be between 3 and 4 feet tall when they do. I binned mine last year after 6 years, got fed up with waiting...

I used to put mine out all summer and just give it frost protection in the winter and it flowered every year, but it was a bit of a mature plant when it was given me

I suspect they do rather better if they spend some time outside in summer at least.

I always place outside in summer. They need heat and sun for Strelizia to do well. Obviously bring inside before frosts, better to bring in in October before temperatures fall too low. Soil on the dry side all winter. They can take a lot of watering in summer. In South Africa they are grown widely outside, even in cooler regions and with irrigation. 4-5 years from seed sounds right. In the UK I keep them pot bound as that forces flowering due to the stress. Be hard to get flowers.

I normally don't comment on how to grow things in the UK, and this plant is one of the reasons why. Mine are in flower now, and they grow outside. So wet winter and drier summer, but the temperatures rarely go below 9 or 10 C. They get enough water naturally, so I don't water them at all, even during the very dry August period. It can take up to 7 or more years to flower if grown from seed. I also have the S. nicolai that has gone 10 years without starting to flower, but I don't think it gets enough sun (they need full sun) and now it is too big to move (well over 20 feet tall).

I do like S. nicolai, with those wonderful white and blue flowers and large musa leaves, but it gets too large to haul inside every winter to be worth it and as not hardy does not flower in the outside in the UK. Strelitzia are part of the Zingiberales genera with Cautleya, hedycium, ensete and musa - all of which you can grow in the UK with winter protection.