How long can I keep a 2.5" external plugged in?

Yes, kind of. But not to worry that much. Temps inside casing are much warmer. But i sense you want portability much more than anything. So get it. and warranty is always there to protect you, so why fear? (unless you don't backup  )

i kept a portable connected for over a year and never had any problems with it

2.5" drives are typically slower and use less power. Considering it will connect via a terribly slow interface like USB it's not likely that there'll be a significant difference in performance between the two because the interface will be the performance bottleneck.

Utter rubbish hard drive i will never support it again because they do not honor their warranty. I have owned 640 GB 2.5" & 1 TB 2.5" and looked after them properly but they ended up having bad sectors. I bought my drives from 2 different locations of the world,USA & Philippines.Both looked after very well, but i ended up having issues with Cyclic Redundancy Check.Lifeguard Tools reported bad sectors and it suggested that it may be able to fix the errors but it was unsuccessful. sorry its junk.

Im sorry they dont last, i have tied to write zeros and all that junk, its nonsense.
either u are lucky or not.

I have had rubbish warranty support and i wish to never buy this brand again.

the worlds worst selling hard drive.

I really dont understand it.