I used sav a chik for 2 days for the first three I got that were very stressed from shipping. I gave it to them for 3 days until they seemed pretty active. When the replacement chicks from that order came in a week later, they were much more lively than the first crew (they had a day less of transit time) and I didn't add any sav a chik. I would have used it if anybody seemed run-down, but didn't feel they needed it. They are all just under 6 weeks old now and are doing well.
I would go on your instincts and watch the chicks. I don't suspect there'd be any harm in spiking their water for longer, as long as you are mixing it weak anyway. But if it's half strength, they probably aren't getting much benefit from it. If you have a packet left, go ahead and use it up, but if everyone's perky, I wouldn't run out and buy more. Just my opinion.