Not very much compared to global standards, according to this Economist article. As they note,
Government revenues average about 17% of GDP in sub-Saharan Africa, according to the IMF. Nigeria has more than 300 times as many people as Luxembourg, but collects less tax. If Ethiopia shared out its tax revenues equally, each citizen would get around $80 a year. The government of the Democratic Republic of Congo is so penurious that its annual health spending per person could not buy a copy of this newspaper.
Globally, wealthy countries have tax revenues equivalent to about 34% of GDP, and Latin America averages 22%, according to the OECD.
One of the paradoxes of taxation in many African states is that low tax revenues co-exist with relatively high tax rates, because the overall tax base is very narrow. Most economic activity goes untaxed, and taxes are concentrated on a small number of firms in the formal sector. And the largest firms, or those which are politically connected, can often negotiate further tax breaks, further narrowing the tax base.
(H/t to Ken Opalo for this article.)