How long does DNS take to propagate and short bash script to test after changing DNS entries

I was changing a customer's DNS to point at a new public IPs. They were concerned about how long it would take to switch over. The DNS provider told me they usually tell customers it will be complete in their network within 24 hours and they can't control outside of that. They said normally the DNS change will take effect immediately and should have propagated out across the internet within 24 hours so this covers them with customers.

Wrote a short bash script to do an nslookup so we could see the new public IP's and a curl to show the website was up.

echo "**********************************************"
echo "nslookup on my.customer.com"
echo "**********************************************"
echo " "
nslookup my.customer.com
echo " "
echo "**********************************************"
echo "curl on my.customer.com, expecting http 200 OK"
echo "**********************************************"
echo " "
curl -IL my.customer.com
echo " "