The last bear market lasted for 17 months, during which total cryptocurrency market capitalization fell by ~76% from its all-time high. If you'd invested $400 in BTC and $400 in the bottom 20 alt-coins (by market cap), then as of today's date, your investments would be worth ~$11,564 (BTC) and ~$18,740 (alts)--a total of ~$30,304--despite the fact that 16 out of the bottom 20 alts disappeared in the intervening years. I believe the lesson here is to spread your alt-coin investments out.
According to CoinMarketCap, during the earlier bull market, total crypto market capitalization peaked on November 30, 2013, at ~$15.2 billion. Thereafter, a long bear market began, during which total market cap bobbed up and down for over a year, hitting its first major low of ~$3.6 billion on January 16, 2015--a ~75% drop--and then going back up to as high as ~$4.7 billion in mid-March. Finally, on April 26, 2015, the market hit bottom at ~$3.5 billion--a ~76% drop.

This year, total crypto market cap hit an all-time high of ~$828 billion on January 7, 2018. If the current bear market were to exactly mirror the 2013-2015 bear market (and it won't, but this is still a good jumping-off point), then we would expect to see the market hit a low of ~$196 billion by mid-June 2019.

As I write this, we're currently at ~$260 billion (i.e., down ~68.5% from ATH), having hit a low of ~$248 billion in the past few weeks (i.e., down ~70% from ATH). This suggests that we may still have a significant distance to go--both in terms of time (i.e., another ~14 months) and market cap lost (i.e., ~25% of the current market cap)--before we see the bottom of the current market. I'm personally somewhat more bullish than that, as I think any correction will be over by the end of Q3 2018, but that's what would happen if this crash were to exactly mirror the last one.