Good info in this thread. I think most is already said just want to add some things based on my experience in offroading competitions.
A typical winch will draw 500 -650 A, that makes that a 100Ah battery will last you 9.2 minutes in theory. Reality nevertheless will leave you with about 2 to 3 minutes of effective power. (due to battery behaviour at these loads)
The problem with this is that once the battery gets weak your winchmotor will burn out.
The winch consumes an amount of power or watts to do its labour (x horsepower represent y Kw), the result is a current, not the other way around. As P=UxI (powerconsumption in Watt = voltage x current ) the voltage drop with an empty battery will cause the current to rise and burn the motor windings.
Why it will harm your electrical system of the tractor:
When the battery runs out of power the load will shift to the alternator. The alternator is not made for loads this big and the spikes will fry the regulator. If that one goes the alternator can give spikes in tension and current to your tractor electrics with eventual damage as a result.
An alternator load rating means nothing more that it can be loaded with eg 60A of users, Not that it can only deliver 60A, If you ask more than its rating it will try to deliver and cook itself.
A fact that is overlooked a lot of times is that for charging a battery you need about 1/10th of its Ah rating. The battery will simply not absorb more. So a 100Ah empty battery needs 10h of charging at around 10A. You can connect a source that can deliver 35A, it will only take 10A.
So running a 100 or 60 Amp alternator makes no difference, it only changes the working time of the winch by a fraction (seconds). Heavy alternators only makes sense when having multiple batteries as this heavy alternator will allow to full-power charge the multiple batteries while running all normal electrics of your tractor.
It would only make sense to upgrade to a 650A alternator which would supply the winch with full power. (and then no batteries needed)
For your own and tractors sake, never install a winch without a high power battery cut-off switch. A short circuit motor will set things on fire quite rapidly and the only way to stop it is cutting battery power. I guarantee you dont have the time to find your key to unbolt the battery terminal.
As an alternative electrical winch choice you can go for a worm driven which, They are slower but "only" draw about 350A at full load pull. These winches are by the worm drive fully braked and can therefore be used to hold a load and lower loads. (planetary winches have a separate brake system that wears out and is not effective under high load. Dont ask how i know but going down backwards an almost vertical slope is not funny that i can tell you) They also dont get as hot (because of less current) so can work "all day")
Look at my powerplatform thread where i am building a 3pt winch platform with own batteries and generator, so separate from my tractor electrics based on a worm drive Husky winch.
If you need the winch for work i would suggest only 3 types:
electrical: worm driven
Hydraulic
or
Mechanical forestry winch
Hope this is useful info.
