First, pull up a Magelo profile. Let's use my cleric's profile since he is about the right level and has no AAs to speak of - http://eq.magelo.com/profile/517490 . Now click on "Modifiers" on the right hand side. The "General" tab will be highlighted at the bottom of the screen. Now click on "HP+" on the left hand side. The Total row at the bottom will say "0/37", so you can get up to 37 HP/tick from items at level 66. Similarly, click on "Mana+" and you will see "0/15".
Checking some of my other characters, Magelo shows the following numbers:
Level - HP regen from items
30 - 35
40 - 35
49 - 35
65 - 35
66 - 37
80 - 65
In other words, it looks like maximum HP regen from items (without AAs) is 35 up to level 65 and increases by 2 every level until at least 80. Maximum mana regeneration from items, on the other hand, appears to be stuck at 15.
Next, click on the Alternative Abilities icon on the right and select "General" on the left. See "Energetic Attunement" 0/15? Hover the mouse pointer over it and it will explain that every AA point spent in this area will increase the maximum amount of regeneration from items by 1, so if you max out this AA line, you will be getting extra 15 HP/tick from items.
Under Archetype, you will find "Expansive Mind" 0/20, which works similarly and allows you to increase passive regeneration from items by up to 20 per tick. Of course, since it's archetype-specific, your class may have a different limit.