I was worried about milis overflow as I am using Arduino Mega as a main controller in my Home Automation but I have read that I should be fine. Just in case I set up logging around 48 days of uptime and those are the results...
Uptime 4294932 49 days, 17 hours, 2 minutes, 12 seconds
Uptime 4294933 49 days, 17 hours, 2 minutes, 13 seconds
Uptime 4294934 49 days, 17 hours, 2 minutes, 14 seconds
Uptime 4294935 49 days, 17 hours, 2 minutes, 15 seconds
Uptime 4294936 49 days, 17 hours, 2 minutes, 16 seconds
Uptime 4294937 49 days, 17 hours, 2 minutes, 17 seconds
Uptime 4294938 49 days, 17 hours, 2 minutes, 18 seconds
Uptime 4294939 49 days, 17 hours, 2 minutes, 19 seconds
Uptime 4294940 49 days, 17 hours, 2 minutes, 20 seconds
Uptime 4294941 49 days, 17 hours, 2 minutes, 21 seconds
Uptime 4294942 49 days, 17 hours, 2 minutes, 22 seconds
Uptime 4294943 49 days, 17 hours, 2 minutes, 23 seconds
Uptime 4294944 49 days, 17 hours, 2 minutes, 24 seconds
Uptime 4294945 49 days, 17 hours, 2 minutes, 25 seconds
Uptime 4294946 49 days, 17 hours, 2 minutes, 26 seconds
Uptime 4294947 49 days, 17 hours, 2 minutes, 27 seconds
Uptime 4294948 49 days, 17 hours, 2 minutes, 28 seconds
Uptime 4294949 49 days, 17 hours, 2 minutes, 29 seconds
Uptime 4294950 49 days, 17 hours, 2 minutes, 30 seconds
Uptime 4294951 49 days, 17 hours, 2 minutes, 31 seconds
Uptime 4294952 49 days, 17 hours, 2 minutes, 32 seconds
Uptime 4294953 49 days, 17 hours, 2 minutes, 33 seconds
Uptime 4294954 49 days, 17 hours, 2 minutes, 34 seconds
Uptime 4294955 49 days, 17 hours, 2 minutes, 35 seconds
Uptime 4294956 49 days, 17 hours, 2 minutes, 36 seconds
Uptime 4294957 49 days, 17 hours, 2 minutes, 37 seconds
Uptime 4294958 49 days, 17 hours, 2 minutes, 38 seconds
Uptime 4294959 49 days, 17 hours, 2 minutes, 39 seconds
Uptime 4294960 49 days, 17 hours, 2 minutes, 40 seconds
Uptime 4294961 49 days, 17 hours, 2 minutes, 41 seconds
Uptime 4294962 49 days, 17 hours, 2 minutes, 42 seconds
Uptime 4294963 49 days, 17 hours, 2 minutes, 43 seconds
Uptime 4294964 49 days, 17 hours, 2 minutes, 44 seconds
Uptime 4294965 49 days, 17 hours, 2 minutes, 45 seconds
Uptime 4294966 49 days, 17 hours, 2 minutes, 46 seconds
Uptime 0
Uptime 1 1 second
Uptime 2 2 seconds
Uptime 3 3 seconds
Uptime 4 4 seconds
Uptime 5 5 seconds
Uptime 6 6 seconds
Uptime 7 7 seconds
Uptime 8 8 seconds
Uptime 9 9 seconds
Uptime 10 10 seconds
Uptime 11 11 seconds
Uptime 12 12 seconds
Uptime 13 13 seconds
Uptime 14 14 seconds
Uptime 15 15 seconds
Uptime 16 16 seconds
Uptime 17 17 seconds
Uptime 18 18 seconds
Uptime 19 19 seconds
Uptime 20 20 seconds
Uptime 21 21 seconds
Uptime 22 22 seconds
I hope someone will find this helpful
Also one more time I would like to thank @blh64 for help in topic Delay in reconnect() blocks execution of other code - #8 by blh64