My/Our WU consists of 3 sets of seated pike and straddle leg lifts. Lately it's been 5 lifts with a 5s hold at the end. Another variation was lifts in 5-3-2 with holds of 3s and we used to also employ 10 lifts with a 3 or 5 hold as well or 3s lifts with longer holds.
Every 3-4 days? For my guys, they do it at the end of every workout which means MWF. I do it at the end of every workout, but only work straddle-L. Generally I do 6x10s. Lately, I've been doing it in an undergrip on the edge of a beam because it's similar to rings though it does flex my wrists hard (I probably should just do it on rings).
For the guys, they need to master both the L-sit and straddle-L sit because it's in the routines (there is the L-hang and L-sit in L4 besides the press from straddle-L to L-sit in L4). In later levels, there is the endo roll press HS besides the need for L strength for the HB kip.
However, it's just done as finish 60s of the hold, mainly cause it's the end of the workout and they vary in how tired they are from day to day and because it's been quite a long time since we've tested hold times (since we have been in pre-season/season now for 6mo). As well, I've noticed these boys tend to suck with ever doing 50% of max time holds and rarely ever want to hold anything more than 3-5s.
I used to try to work V-hangs and straddle-L holds for time and that would just crush my hip flexors. Now, I just incorporate the V&L-hangs in HLL. 3s in V, 3s in L.
3 of my guys probably have the flexibility for manna one day but do not have the strength yet as they are far off from optionals. I'm thinking it would be a good goal to get them a manna by the time they are in optionals.