How much returns should I be expecting if I invest in ETFs?

The returns you should expect depends on the type of investments the ETF tracks or holds. For instance if you own an ETF that focuses on high-dividend stock, you will probably earn the capital gain and dividends paid out by the stocks the ETF tracks. The ETF should perform roughly in line with their underlying holdings. You can look at an ETF's tracking error which measures the volatility in the difference between the ETF and the underlying index.

An ETF is just a fund.

You need to state which ETF, for how long, DCA or Lump Sum.

If you manage a global portfolio, whether ETF or UT, you can predictably achieve 6-8% returns per annum over a minimum 10 year period.