I have been using the standard 90 days for the spell when I enter the rules into Bet Selector.
And I use the 4% of bank for liability for staking so I cover all the bets the system comes up with even with $100 odds selections otherwise you are robbing yourself of lay collects restricting to odds under $10.
Admittedly one makes a lot less profit on the longshots but the risk is exactly the same for a longshot or an even money winner with liability betting and all bots I have looked at have the liability staking option.

Example for those that do not know about liability staking
$100 bank betting 4% of bank liability
4/99.0 this is odds minus 1 $0.04 = stake. profit= $0.04. liability = -$3.96
another example
4/1.00 = $4.00 stake, so profit = $3.80 after 5% commission is deducted and liability = $4. Betfair does not deduct commission on bets less than 10 cents because 5% works out to be less than 1 cent.

So you can see the liability is the same when the stake is worked out to the nearest cent. Liability is the amount you lose when a lay bet wins and you have to pay out to the odds less one.

On real longshots where the stake works out to be less than one cent of course no bet is placed ie 4/999 =0.004, this is for the longest odds you can lay/back on Betfair ie $1000 and the shortest is $1.01.