You can invest your entire discretionary income if you want. But it would be better for an investor to invest 25% of his discretionary income. If a person uses 25% of his discretionary income, then he can build an emergency fund with the remaining 75% and if the emergency fund is ready, then he can keep that money with him and when he sees a decline in the market, he can buy aggressively.

I think that with time an investor does everything. Maybe a new person may not do this, he may not be aware of this. But a person who has been investing for some time may set aside money to buy aggressively. Because he knows how to use the decline.