Cap Questions

Just keep it plugged in while playing and you'll be fine 

All capacitors work this way. And all but the most unusual amp designs use them. Naim provide much larger reservoir caps than normal. I wouldn't worry about the caps specifically so much as whether a 250 is suitable for your speakers in general (which your profile doesn't mention).

At high volume the sound will deteriorate immediately if the amplifier is not powerful enough or if the loudspeakers are not efficient enough. Nothing to do with caps. 

Yes thy do. The only way you will notice how they discharge is when you switch your 250 off - it will take it approximately 1 minute to be completely OFF

Speakers are SBL. 52 Pre, streaming from laptop to Rega DAC (USB). So how long can I push the amp before the sound quality begins to be comprimised?

Just use it for as long as you like and it will be fine. The SBL is an easy load and your ears will give up long before the 250. Should it ever overheat, it will shut down until it cools down.

One word of warning though - has the amp been recapped in the last ten years? If not, get it done. I left a 250 a bit too long once and Naim told me that if I'd left it any longer it would have failed and taken the speakers with it. 

I've heard caps running out of charge when I had a Maia power amp, 80W rated the same as a NAP250. The speakers at the time were Thiel CS1.6 which dipped down to 3Ohms at 5kHz. The music was Simon Rattle and the Berlin Phil playing the performing versionof Mahler's 10th which has an unusually sustained forte passage and I had it cranked up a fair bit. The sound seemed to sparkle and jump forward out of the speakers in a very odd manner. Those speakers were on home demo, though I subsequently ordered a pair I was a little more careful with the volume with my pair, until I replaced the Maia with a 250, they would drive those speakers through that passage at even more elevated volume without a hint anything sounding amiss.

 Amp is recapped and so is 52PS...so I am good to go! Thanks HH.

You most certainly are. The 52/250 and SBLs are truly wonderful, but do bear in mind that you really should be looking at a significant source upgrade. While wonderful, the system is also mercilessly revealing.