I don't buy tax liens myself but I do help my clients foreclose and can explain the legal process. So any expenses you incur after about 4 months from the date of the tax sale become reimbursable (attorneys fees, title search fees, etc.). At that time you can hire an attorney.
First thing I do after getting hired is perform a title search to see who has interest in the property. Then I send them the required notices to file a complaint.
Then, after the notices expire (after about two months) I can file the complaint to foreclose the right of redemption.
In my experience most people actually redeem the property by paying off the lien and my client's expenses before I can file the complaint to foreclose.
Owner's have the right to stop the foreclosure by paying off your expenses and the lien up until the final court order. Which they often do.
For the complaints I've filed last December I still haven't had a court date yet. So the owner's can still stop the foreclosure.
I still think it's worth doing. But I'd only do it if I could buy lots of liens so hopefully 1 or 2 actually result in obtaining a property.