You should be able to replant after 6 weeks. It's safe only if you carefully follow the instructions to a tee.

If you are painting the SBK on the cut stems, it is only effective when painted on within ten minutes of cutting the stem--something that they don't always mention in the instructions.

By far the best way to deal with this is to grub them out with a decent mattock, cut down to ground level and then grub them out, go over the area a few times to be sure all roots are cleared, then leave for a while just to see if any new growth comes up from any missed roots, then prepare for seeding, yes it’s hard work but if you get some help and have a few decent grubbers then you will soon clear the area.

I'm with Julien on this. Yes, it is hard work but a lot quicker if you're prepared to put the time in. You can then seed or turf and then, if you have missed some brambles, then you can paint the few with SBK.

I found that if new brambles appeared through the grass from some leftover roots regular mowing finished them off very effectively without needing to use a weedkiller, as they don't live very long without any leaves.

If its too difficult to dig out all the roots, there is a way to do it that doesn't poison the soil. There is some digging involved though - you will need a pair of sharp secateurs or possibly some loppers. You need to dig round each cut down 'stump' to make a hole and reveal (hopefully) a woody root or three, at least 3 or 4 inches below the soil level, more if possible, cut back to something fairly thick, slash into it to make cuts, or split the root by cutting downwards through the middle, and immediately apply SBK, neat, carefully so it doesn't spill onto the soil. I usually use a small container (like the cap off the bottle)pour a little SBK into that, then apply to the cuts you've made. Then you use as small a pot as possible - any old pot, a cheap plastic one is fine, invert that over the top of the treated area and replace the soil around it and over the top. Move onto the next bramble and repeat the process. You should wear gloves, the latex ones are good for this because you want to be able to feel what you're doing as far as possible, its just to keep the SBK off your skin.

Then you need to wait six weeks I'm afraid, although I'll admit I've often planted immediately - but not grass or seeds, it's only possible if you're planting the area with shrubs or perennials. Often the pots are visible at soil level, just the tops, so obviously, trying to create a grassed area isn't really an option at that stage. The purpose of the pots is to keep soil off the SBK treated area, and to make locating the roots easier if regrowth occurs.

One tip if you do this - use a tray or something to stand the bottle of SBK on - more than once when I've been doing this, its fallen over and has leaked onto the soil, so at least if its on a tray, its only the tray that gets the SBK... and its safer to tip the SBK into something small over the tray too, rather than hovering above the soil. The object of this exercise is to put the SBK only on the cut roots, and not on the surrounding soil or any soil at all.