How long can I run my 1993 subaru with the fwd fuse in?

the users manual covers this in the glove box, if lost

the manuals seem hard to find , free.
lacking that test all fuses.
i use this

https://eautorepair.net/Marketing/Default.asp

at 45 mph it should work

From a Subaru mechanic:
The FWD slot should not have a fuse in it. Remove the fuse and the AWD will work as it should. The slot is there for us to put a fuse in to test the rear portion of the AWD. With the fuse in, it shuts off power to the rear wheels.
Remove the fuse and retest.
From the mechanic I chatted with, the fuse should not be there - except when they are testing the AWD. I would take it out.
He did say that if you run into a problem where the rear drive is not working correctly, you can put the fuse in, but ONLY LONG ENOUGH TO GET TO A SERVICE STATION - it is NOT meant as along term soluution

There is a fuse box in the engine compartment on the passenger side labeled FWD and, under normal conditions, should be empty. The user manual indicates that a fuse can be installed in order to disable AWD. This should be done if you are using the spare tire, the car is to be towed w/o a flatbed and when the front and rear wheels aren't the same.
I imagine that the driver checked this fuse box and found it empty and told you the fuse was blown... does this sound about right?
With the fuse installed you will get a warning light on the dash (red FWD) to remind you that your rig is running in FWD mode...
This FWD fuse box is only present on AT models.
Why was the car towed and did they lift the front end or put it on a flat bed?
Cheers, immorion

The FWD fuse is working the right way from what you describe. The problem with the wheels sounds like a alignment problem.

four wheel drive

It is not recomended to run like that all the time. It is really just for towing. If it seems to be running better I would suspect that either the rear or the viscous is in need of attention. Hope this helps and have a good day.

There is a box labeled FWD under the hood passenger's side by the windshield.  If there is a fuse in the box then this will disengage your all wheel drive. Try removing the fuse

I know this has to do with running the spare tire. since the car is all wheel drive all tires must match (type/wear) the spare does not match those. The fwd mode must be engaged while running spare.

If you have the owner's manual of the car you should be able to check that out, and see which fuse it'll take. Should tell you what type, and what amperage(size) the fuse is. If it isn't in the owner's manual, it may be listed on the fuse panel cover(s).