How long? Depends on the number of figures you use. About 1 hr for a smallish battle (4000 pts- 5 to 7 units) to 3 hrs for a bigger battle (10000 pts – 15 to 20 units).
Table size? I'd say a 6x4 will do just fine in 15mm. A 4X4 will allow you to play all but the biggest games, but the bigger the table, the more maneuver room you will have.
Base size? Its individual basing. For 15mm Thane the rules say 15mm squares(for regular troops- bigger troop types and monsters get bigger bases 30mm square and 45mm square repectively), but you could use anything around that. As long as both armys are based ther same, you could really use anything, but 15mm.
How many figures? See answer to the first question. It depends on how big of a game you want. If you figure each regular unit is about 20 guys, you can do the math from the number of units I listed above to get an estimate. It will, of course, depend on the army you play as the troop types have slightly different costs.
Check out the Thanesgames website for lots of info and links to armies other people have made to get an idea.(Of course 4 of those links are too my armies and they don't work anymore as I moved and lost my old website—need to remember to send the HTML files to Thane) You can also get any questions answered at the active Yahoo group