If I have to vote, these days (when there are not many to vote for) the amount can be as little as several k USD on a small exchange.
For now 30 day is very good.
Exchange risk depends on the exchange. There are many ways one can lose fund accidentally on an exchange, from hacking to exchange haircut (vircurex) or going under (mt-gox). I think all LPCs should stay on reputable exchanges. Of course it is nice to have LPCs posted on every exchange but it is not practical. It makes little sense to sponsor an LPC in a dark alley exchange to be mugged.
For the same exchange, exchange risk is roughly proportional to how long the fund stays on the exchange. For a dual-sided (buy and sell) LPC the fund presumably stays on the exchange for the entire contract length, because fund cycles between the two walls. As a first order approximation, the risk can be estimated by calculating what probability the exchange goes under in the contract period. If we use an exchange that has been around for two years (24 months), the probability for it to go under, causing a total loss of LPC’s fund, in one month is 1/24, about 4%. So if a contract of one month, this part of risk is about 4%.
Single side LPC is different. The operation is basically sell or buy NuBits. When Nubits are sold or bought, the proceeds can leave the exchange. When all are sold/bought, the LPC’s job is done. So the risk can vary between 0% to 4% in the above example.
On top of exchange risk to compensate LPC’s cost 5% can be added. There should be a cap I think. But I would like to hear from existing LPCs.