There are threads on this every year, there are online articles & 'professional opinions' and NO conclusion.
I have a combi now, I want hot water, hey presto-hot water! Just like magic!
It very much depends on your system & your usage.
if you're thinking about the cost, why not scale back how long it's on for each time, until there's suddenly not enough hot water for someone's shower or whatever. Then you can move it back a little bit longer.
or alternatively how long does it take to get hit enough for a shower?
or everyone has shorter showers...
there's no right/wrong answers, just tweaking it for your individual needs, according to your individual system.
combis may have their faults/limitations, but they're SO much easier!!