I wouldn't hesitate in the least to dig a ditch that long. On my place with very little level and many large rocks/boulders hidden in the clay, I'd reserve a weekend but expect to be done in a day.
If the ditch was 6 feet deep I'd rethink the project so that I didn't have to dig that deep, as the PT wouldn't be able to do it. 4 feet deep would be the max I think and probably take 3 times as long as one 2' deep.
I've had no feeling that I was overstressing anything on the machine at any time I've used the mini-hoe. Of course, I've had no feeling of overstressing the machine with any implement. Anytime there's too much weight or resistance in the front, the rear rises into the air. That isn't even a problem as the pucker-factor lift is calculated and predictible. I've even demonstrated it for others, scooping up a large load of gravel with the FEL then 'turning' the PT to change the COG and making the rear float into the air.
My DW always doubles the time and money that I estimate for a project and more often than not she is closer to the actual time/money spent.
Phil