Well 600W (of whatever light? Assuming HIDs, with fullspectrum LED COB you could do 2-3 plants with that) is quite enough to grow a nice big plant, and 40L
can support it (on water only and a few topdressings, if it's a good living soil, I don't have experience with anything else sorry).
But I just saw you said "they", so if you've got more, you need to factor in how much
area you have for them too (my bushes were cramped on 50x50cm each). And if it's bound to get tight, then switch to flower earlier, otherwise I'd let them go for another few weeks. Because that time allows the plant to develop a strong basis for lots of flowers, also you can train them to develop their secondary branches more and get a wider canopy, giving you more evenly developed buds all over.
I've grown NL5xhaze twice to now, once scrogged (the hazey and the indica pheno) and once free-standing (hazey pheno), and the hazey phenos DO stretch
significantly going into flower - which is why this strain can be grown at 12/12 from seed and still make 1m tall bushes, AND why you have all this flexibility to influence their size by switching to flower earlier or later.
So when you switch to flower is going to depend on all that.
HTH