Changing 2nd profession

where can i do this?

Depends upon the campaign.

In Prophecies: Crystal desert. Once you ascend, NPCs in outposts in the desert will give you quests that will let you change your secondary to a specific other secondary (eg. an NPC will offer you a quest that will let you become a secondary necromancer once you complete it). Once you "unlock# a secondary in this way, you can switch between available secondaries at any time.

In Factions: Once you beat Nahpui quarter mission, you will gain access to the area where there is a single NPC that will allow you to change your secondary profession to any other profession. The first time you change to a new profession, he will charge you 500 gold per profession.

In Nightfall: In the Command post, accessed from the Sunspear sanctuary, once you complete a few quests, you will find an NPC that will change your secondary profession. Same rules as above.

how long does it take to ascend?

In the Crystal Desert you have to complete the 3 desert missions (Elona Reach, Thirsty River, Dunes of despair) to be able to access the Ascension Mission from Augury Rock.

As to how long - it depends on you as a player, how fast you progress through the game. Ascension is in the desert, the 5th of 7 general areas of the game (Old Ascalon, Northern Shiverpeaks, Kryta, Maguuma Jungle, Crystal Desert, Southern Shiverpeaks and Fire Islands)

In other words:
Ascension in Prophecies follows mission 20 in the regular mission sequence. Means rather towards the end of the game.

It is possible to break the sequence, though, by getting to the Sanctum Cay mission "on foot" form Lion's arch. In that case, you'd only need to do the 4 desert missions to ascend.

An even faster way would be to get to Lion's arch, travel to Cantha, and only do the first two missions there - Vizunah square (foreign) and Nahpui quarter. That would already get you to Senji's corner, making it the fastest way to ascend.

Very similar would hold for traveling to Nightfall, passing the consulate docks mission and getting to the sunspear sanctuary.

I believe to Ascend you have to do the Asention for the map you were made on so you can't travel from Tyria to do a quick ascend on Cantha/Elona

I could be wrong

I think that's true. My Tyrian born Necro can't get her second 15att point quest because she isn't ascended in Tyria. She is in Factions though, I took her there to level up quick, but it makes no difference. Although, she can change secondary professions due to the Canthan 'ascension'.

You don't have to ascend on your home continent. However, if you want to do the quests in the Crystal Desert that let you change your secondary profession, you must have completed all of the ascension related missions in the desert. Exactly the same applies to the second attribute quest in Tyria.

Just tried a couple days ago to use the profession changer in NF-Command Post for a Prophecies born character who hasn't done the Ascension quests......couldn't change. The dialogue says "You must be Ascended or Closer To The Stars to change profession"