Regular market hours trading sessions Pre-market, after-hours, and overnight trading sessions
Orders can be placed at any time and will only be executed from 9:30 a.m. to 4:00 p.m. (U.S. ET). Orders in extended hours can be placed outside of regular U.S. market hours (9:30 a.m. to 4:00 p.m. U.S. ET) and are available for the following times:

For orders placed on thinkorswim platforms:

7:00 a.m. to 8:00 p.m. (U.S. ET) with five-minute closures before and after regular U.S. market hours.

Overnight trading is available 24 hours a day, every market day, by choosing an EXTO order type. EXTO orders expire at 8:00 p.m. (U.S. ET) each day. For example, an EXTO order placed at 2:00 a.m. (U.S. ET) Monday morning would be active immediately and remain active from then until 8:00 p.m. (U.S. ET) Monday night. A trade placed at 9:00 p.m. (U.S. ET) Monday night would be active immediately and remain active until 8:00 p.m. (U.S. ET) Tuesday night.

For orders placed on the Schwab website or Schwab Mobile: 

7:00 a.m. to 8:00 p.m. (U.S. ET) with five-minute closures before and after regular U.S. market hours.
Trading occurs on exchanges like the New York Stock Exchange (NYSE) and Nasdaq and through a variety of venues, including market makers and other market centers. Similar to regular market sessions, trading occurs on exchanges like the New York Stock Exchange and Nasdaq and through a variety of venues, including market makers and other market centers.
Many order types and conditions are accepted. Including: market, limit, stop-limit, all-or-none, etc. Only limit orders are accepted.
Generally, all order sizes are accepted. For pre-market and after hours trading sessions on Schwab.com, there is no maximum quantity on a single order. 

For an overnight trading session, 50,000 shares, or a notional value of US$10 million dollars, is the maximum quantity on a single order.
Many security types are available. Including: stocks, options, bonds, mutual funds, etc. Most listed and Nasdaq stocks and ETFs are available in pre-market and after-hours sessions. 

The overnight trading session is available for select securities and exclusively on thinkorswim platforms. (Currently available securities are listed in the "24 Hour Trading" watchlist.)
Different timing choices are available. Including: Day, GTC, IOC, and FOK. Pre-market, after-hours, and overnight session order types are only good for the particular session in which they are placed. Seamless orders that participate in pre-market, regular market, and after-hours trading are available to be placed GTC. 

Note that GTC EXTO must be chosen to place overnight trading in thinkorswim trading platforms.
In general, higher-volume trading activity means more liquidity and a greater likelihood of order execution. Lower volume trading activity may result in lower likelihood of order execution, plus wider spreads and greater price fluctuation.
The quotes you receive are consolidated and represent the best available prices across all trading venues. Market makers work to ensure fair market pricing so that customers get the best available buy or sell prices. Quotes and fills are not consolidated and represent the current prices available through the market. As a participant in the extended hours trading network, the market may also offer access to prices available in other participating markets but not necessarily all venues open for extended hours trading. Best execution is not guaranteed.