The expense ratio is stated as an annual figure but is often broken down to be taken out periodically of the fund's assets. In traditional mutual funds, there will be a percent of assets in cash that can be nibbled to cover the expenses of running the fund and most deposits into the fund are done in cash. In an exchange-traded fund, new shares are often created through creation/redemption units which are baskets of securities that make things a bit different. In the case of an ETF, the dividends may be reduced by the expense ratio as the trading price follows the index usually.
Expense ratios can vary as in some cases there may be initial waivers on new funds for a time period to allow them to build an asset base. There is also something to be said for economies of scale that allow a fund to have its expense ratio go down over time as it builds a larger asset base. These would be noted in the prospectus and annual reports of the fund to some degree.
SPDR Annual Report on page 312 for the Russell 3000 ETF notes its expense ratio over the past 5 years being the following:
0.20% 0.20% 0.22% 0.20% 0.21%
Thus, there is an example of some fluctuation if you want a real world example.