I am not a vet or an expert, but here is my take on your question:
I never let the calf go more than an hour in winter and never more than two hours in warmer weather.
A calf that just lies there and doesn't get up will actually affect the cow's mentality. She will begin to loose interest in the calf and give up on it if it does not respond to her attempts to get it up and nursing.
When I give Colostrum, I only mix about half a bottle, or if I use the cow's milk I still only give half a bottle. You want the calf hungry and wanting to get up and get more of that 'good warm stuff'. Giving the calf too much satifies it and makes it less likely to try and get up. I have had calves try to get up after just one or two sucks on the bottle. It seems to give them the drive.
After 24 hours (I think) the linning of the stomach changes and the calf can't digest colostrum. So, don't go to bed thinking the calf can wait until morning. By then the calf could be very cold, the cow could have lost interest and the calf has less of a chance digesting the colostrum.
As far as tubing, I consider that a last resort. The calf needs to use it's sucking reflex and there are risks to tubing if you are inexperienced. I am a retired nurse anesthetist and have put more tubes into people in places we don't want to talk about, but I still am hesitant about tubing unless I have to.
Good luck and hope the calf is OK.