Without any rests, indication of time signatures, or clues about rhythm I think it is a fairly safe bet to assume that this song is in 4/4 and most of the notes are quarter notes. In the bars with fewer notes tabbed we are likely looking at whole-notes or half-notes which likely fall on the 1 and the 3. So bar #20 you'd play a
5 on the 1-beat and hold for 4, bar #22 you'd play your
2 on the 1-beat and the 3-beat. That's how I would play it anyway.
There are likely an infinite ways to pick this apart if you really wanted to but it looks like a fairly straightforward rock line.
Alternative #1: Radical Time Signature Changes - if you like you could treat every note as the same length and assume that bar #20 and #24 are a quick switch to 1/4 with #22 and #23 being a dip in to 2/4. Very math-rock ;)
Alternative #2: Guess The Rests - If we assume all notes are the same length (all quarter-notes), and all bars are the same length (all 4/4), then there are some implicit rests (r). So bar 20 you could play as
But if in doubt I'd stick with my original suggestion ;)
EDIT: Having listened to a few seconds of the piece that Topo has linked in his answer, yes if this is indeed the piece of music it does appear to be a fairly straightforward 4/4 piece largely in line with my original suggestion.
BONUS EDIT: Really, the best thing to do is play with the music and see how you like to play it, how it feels under your figures. Obvs if you're playing this in a group you have that constraint but feel free to experiment where the notation is incomplete :)