About as long and as tough as you want it to be.  The first time through I wouldn't even worry about battle mastery's just get used to the system... and if you get stuck.

You can let units die unlike Fire Emblem...(And units WILL die.) and if you lose you even keep all your EXP and money.

So, you'll win eventually.  I know one of my favorite things to do is to enter every battle with the weakest units I have available... because i'm the guy who likes to use everybody.

That's one of the things that would help you out.  It's got a lot of replayability like that. 

Wildschwein

R-blade

http://www.youtube.com/watch?v=8B-Lg7adDDA

That was my favorite in SRW OG.

Oh crap. Super Robot Wars OG gaiden the PS2 has a Free battle mode? Is that the only game to have one?

All this makes me want to get a Japanese PS2 and the console games.

Definitely, I had it down to a science by the end of OG.  I'd attack with the stronger units and finish with the weaker units.  The XP/Money trick is really cheap though... especially in scenarios where you can make a ton of both.

I like Real Robots more so than Super Robots or rather... I don't like the normal Grungust models (like Irm pilots) and robots like the combattler look so... dorky.  Though Sanger's Grungust was awesome because of his sword... and because he's Sanger.  I actually kind of missed the old Type O after he got the Daizengar.

My favorite robot has to be the Rein Weiss Ritter.  It is quite possibly the niftiest looking robot in existence.  It also has the added advantage of probably the longest range in the game (next to the Wildfalken) and my favorite character as its pilot.  The battle taunts for Howling Rifle X and Oxtongue Rifle D were hilarious.  ^_^

Sadly youtube lacks videos of it... though here's one:

It's a combo attack between two mechs
'
The Horse's name is Trombe and is a mech itself, that transforms into a horse. Or the Aussenseiter. Depends who you listen too.

While the Other mech is the Daizengar. I think it had a different name in english though... maybe i'm wrong though.

http://www.youtube.com/watch?v=2KPhxQU-wAQ

That robot is the Daizengar also known as the Dygenguard short for Dynamic General Guardian in the English OG releases. It is piloted by Major Zengar Zombolt (Sanger Zanvolt).  It is the next robot Sanger pilots after discarding his Grungust Type O (3) in SRW:OG2.  

The second robot is the Aussenseiter or DaiTrombe which is piloted by Elzam Von Branstein known in SRW:OG2 by his alias, Ratsel Feinschmecker.

They combine together when initiating a special attack called the Tornado Blade which can only be used when they're near each other.

Every robot can have its abilities upgraded (HP, EN, Mobility, Armor) and depending on the robot's maximum equipment points you can add several different weapons to them in addition to their standard armaments.  And you can upgrade all weapons (default and added).  That being the case, it's really really hard to just pick a single powerful mech since the most powerful mech in any game could easily just be the one you spend the most money upgrading.

 Not to mention that the pilot really makes the mech.

That's another fun thing to do if you really like the series.  Swap out a lot of the pilots with each other.

Espiecally with Excellen.

The OG character personalites are golden. 

I used to have a low opinion of the Gespenst (base SRW:OG mechs), until I realized Katina's power in one.  The Jet Magnum is pretty powerful and though has a will/morale requirement, it also has a really low EN consumption rate.  Once Katina gains Drive, she can instantly fulfill the will/morale requirement and she suddenly has a very powerful weapon at her disposal that she can use every turn without fail.  Give the Gespenst a Tesla Drive to make it fly and it becomes even nastier.

Excellen is great.   Here's a quote from a Wiki I like (that I eventually want to see in-game): 

When using the Giganscudo's Sheath Thunder attack, Excellen quotes Back to the Future by saying: "1.21 Gigawatts? Great Scott!"