Turkeys take around 20 minutes per each lb of the Turkey to cook so the time it takes to cook your 25 lb Turkey would be 500 minutes.
So since there are 60 minutes in each hour that would mean it would take around 8 hours and 30 minutes to cook a 25 lb Turkey.
When cooking a turkey you should thaw it out in the refrigerator for 24 to 48 hours prior to the date you're gonna cook it.
You really want the Turkey to be thawed so that it cooks all the way though.
If you're cooking the Turkey in a Turkey Fryer you want to make sure the Turkey is thawed because putting the frozen Turkey into hot oil can cause a flash back fire.
The Turkey when thawed will cook in the oven or the Turkey Fryer much better.
If you have a meat thermometer you can stick the meat thermometer into the thickest part of the Turkey and when the temperature of the Turkey is 165 F the Turkey is done and ready to eat.
If you do not have a meat thermometer you can tell if the turkey is done you can look at the clarity of the juices.
When the Turkey has been fully cooked and is done there will be no more pink in the juice and if there are some pink in the juices the Turkey should be cooked a bit longer.