Does that mean the spell will work until I'm lvl 30 or does that mean it works on enemies that are lvl 30 or lower.
Also, the conjuration spells stop at "works on enemies up to lvl (40)" does this mean that pas lvl 40 all my conjuration powers are useless as the most powerful conjuration spell doesn't go beyond lvl 40?
Or will conjuration still work when I'm lvl 50 even though the spells don't go that high?

It's the enemies level not your own

I believe it means that any enemy above level 30 are deemed too strong and powerfull, and simply shrug off the spell or have more than adequate resistance to the spell or item etc.

Anything 30 and below you'll fry them with the spell!

Hope that helps, cheers.

Jaeger.

It's the level of your enemies.

I'm not sure how to work out exactly what level your enemies are, but I think they're either a few levels above or a few levels below you, depending on where you are.