I pull one or two shots in the mornings with my Cremina. I like to keep the water fresh, so I try to keep the boiler about half full. It heats up faster as well if it's not very full. Do you fill to top, use it all and re-fill or re-fill more frequently like I do?

I always fill mine to about 1cm shy of completely filling the sight glass view port after I'm done.

Never use all the water unless you want to fry your element, if not your wiring... in fact don't ever pull shots below halfway point. I find the steam performance is best when boiler is about 80% full and by the time the boiler level reaches halfway, the steam performance has waned and the brew temps has risen considerably. This is due to the fact that it takes more time for the boiler pressure to trip the pstat, and in the process heats the water to a hotter temperature range, so your brewed espresso will taste totally different. Try pulling shots with a full boiler and decide if you like it better or not. My water temp at the start of my session is usually between 196 and 198 (measured right below the showerhead). By halfway point it had climbed to about 203-205, much too hot (the espresso gets muted, high notes are greatly subdued).

Two thirds full at every use after which I empty. Simples..